There's some decent info here and here about using acids in brewing. Most AG brewers use lactic not only for the twang but to lower the pH of the mash (in lieu of the crap shoot that is a true sour mash).
In extract brews I've heard of people adding it anywhere from flameout to bottling, though if it were me I would probably add it at bottling if I wanted it solely for flavor (treating it like you would a flavor extract).
It can get too sour pretty quickly and 5mL is about 1 tsp, so you may want to start by adding 1/4tsp, gently stir it in, taste and add more 1/4tsp at a time until you like what you taste.
As far as whether or not you should still use it, that's really up to you. If you like the tartness/sourness of Belgian Wits and Berliner Weiss and you want your IPA recipe to incorporate that flavor then you certainly should.