Basically you'll want to have your mash between 145-158 F. In general, the higher the mash, the more unfermentable sugars you'll have in the wort and the lower, the more fermentable sugars you'll have. This means, generally, that the higher the mash temp the more body associated with the end beer. If you are trying to hit a real low FG, go lower, if you want a higher OG mash.
FWIW I tend to mash between 148 and 154ish depending on what I'm brewing. For a IIPA, I would probably mash low since you'll want to dry it out.