About DoT skills

Damage over time is generally best used in situations in which it is likely to run near to full damage and/or when it's important to minimize the chances of drawing aggro towards you. As a general rule I wouldn't advice using DoTs on your work horse macro, although they usually do amount to very decent damage in even relatively short fights. Blazing/Noxious is generally a good opening combo on scenarios in which you're very likely to draw hate (eg a phys resistant mob and a low dps/underskilled pet).

Blazing Scarab - It's got a low mana cost and a fast casting time, this is your go to DoT. I'd recommend this for boss fights, especially when pet tanking. You may use it on ressilient mobs, although it should be rare for a fight to go long enough to make it worthwhile.

Noxious Gas - Your go to AoE, avoid using this against single targets unless you really need the chi or are actively trying to keep your aggro down. The short range allows for relatively safe use on such situations. Usually you need to apply at least one pet skill to a mob to keep it from being drawn by this, although if the pet is directly attacking it that should be enough. This is per use, meaning you have to reapply pet skills in order to use it a second time.

Frost Scarab - Not worth it for leveling venos, this does have some PvP endgame aplications and a few players find some specialized PvE uses, but for most of us it is simply too expensive (in terms of chi) to use efficiently.

Myriads - I wouldn't really consider the DoT effects of both myriads to be the deciding factor when using them. These become work horse skills as soon as you get them.

It depends on your play style. MANray made some valid points, so take note of them.
Personally, I kept my dots leveled up for pve reasons. I love using them on archer mobs and mobs that run away, as lucky scarab's stun is too short to always be effective at dding. Also, I use them on boss fights, when the dots really are worthwhile. A simple rotation of ironwood-lucky-venomous isn't on my liking (I get a twitch when i see venos spamming venomous ad nauseam).
Noxious does some really nice damage and it is one of my main skills, even on single targets, but only when the fight will drag on (elites, higher lvl mobs etc.) and I have a pet tanking. If you wanna dd your heart out, direct damage is preferred.
Frost...hmmm, idk, I really like using it on fire mobs, especially since at higher levels the dot is so much more reliable. I also don't use nova as soon as I have 2 sparks, so frost is pretty nice. I'm really excited for the sage version I got recently, it is sooooo much nicer.
However, level your main skills first, I think I was well in my 80s when I got the dots to lvl 10. You don't use them that often, so save your coins.

Thank you Kiseop, I'm glad that was helpful. b:cute

A couple of precissions.

@DeffyNature - You're absolutely right, it does come down to playstyle. I think because I wanted to provide some pointers on use when soloing multiples I may have missed pointing out that my recommendation when it came to Noxious was in large part directed towards squad game play. A full 6 person squad should bring even elite mobs down in no more time than it takes you to hit 2-3 scarabs. In this scenario casting Noxious vs single mobs is very inefficient, it takes you twice as long as anything else and there really isn't time for the DoT to kick in. Unfortunately I see a lot of venos out on the field doing this on a regular basis, I pressume to keep their chi up, but even if you discount the boost Ironwood gives to melee your basic contribution should be raw damage, which using Noxious cheats your squad out of. In the scenario you describe (pet tanking) Noxious improves considerably if used early on, but personally I still prefer a more conventional approach. I usually kill mobs in 3-4 hits, my macro up to that point goes Venomous - Ironwood - Venomous - Lucky. The reason is Venomous remains a good, fast attack for half the mp other scarabs use, and I prefer it as an opening salvo because I've found Ironwood in this position tends to lead to chi management issues. Finally Lucky as a fourth hit tends to land at just about the right time a stun will be useful in case an early crit draws aggro away from my pet.

Now as for Frost, you do make a great point about kiting mobs and I can imagine it comes handy in this scenario. Personally I tend to lure them using Holy Path (eg TT1-x) to position myself at a safe enough distance i can resummon without taking hits, but this is personal preference. Now, traditionally Frost is not favoured as damage because you're better off sparking and hitting three times with a multiplier in terms of full output, plus the effect is generally seen as only marginally useful for veno. However much like you I favor not hitting Nova every time I get two sparks, for one chi demands on Veno can be difficult to meet if you're using Lending Hand, keeping Ironwood up and casting Myriads, but also against single targets I believe you're better off sparking in terms of final damage output. I very much agree with your advice on skill leveling, I maxed to level everything outside the fox skill trees (except for Frost and Tame but including Fox Form, Amplify, Purge, Leech and Stunning Blow) although this came at a great cost, I did keep myself from leveling for this and other reasons, and I'm currently struggling for SP. Your average Veno is simply not going to have enough spirit to get this done at my level range.

@Krisnda - I think I may not have made it clear that I consider using Myriads a must (this is what I meant when I described them as work horse skills) but the point I was making was more about the DoT component. While the chance for 2 DoTs hitting for 4.5k hp each at instacast is pretty good, it simply isn't worth 800mp on its own. Before I started regularly using Myriads I could mostly recover from an average boss fight by simply using Nature's Grace, now I need to constantly pot and transfuse to even make it to the end without my mp running out. The kills do speed up, what makes myriads worth using is the chance for armor/mind break, as you yourself point out.

Thank you both for nuances/additions. b:thanks

Eh. I don't like frost if it's not demon. Blazing is nice for the chi gain but for PvE frost just burns chi that can be used for better things and in PvP it's a waste of a spark for crappy damage. Demon frost being the exception due to it giving a freeze that can be used outside fox form.