My own general salvage rules:
- whites, blues, (cloth)greens - Basic 25 use kit.
- Rares - Yellow Kit
- Exo - BL Kit
Edit: http://wiki.guildwar...tic_Salvage_Kit Will save you a decent bit of money on Rare kits, if you have mystic stones spare.
I agree with the above, with the caveat that I generally salvage any leather that won't result in tier 5 (Thick) leather.
It's worth noting that in general, lower-level crafting materials hold quite a bit of their value due to the large quantities of them needed for ascended crafting. Rugged leather (T4) is actually the most valuable at the moment, Thick leather is the cheapest. Almost everything is worth more than mithril. On the other hand, every type of cloth is worth salvaging for.
Naturally the cheaper the harvesting tools you use, the better your margins are on the materials you harvest. But a hundred swings of a 4-silver orichalcum tool is easily going to pay for itself no matter what you harvest, it's just a matter of how highly you value convenience.