just depends on how you're lifting eg realyl heavy long breaks maybe justa few times just to keep eveyrthing alright but def thru your first 2 or 3 exercises as you start to pump blood into the area. take biceps for example. there is onyl so much blood that can be pumped in there. start stretching it and you can kind of help "open up" more area if that makes sense, for more blood to be pumped and a better pump acheived. hiope that doesnt make me sound like an idiot lol i dunno how to explain. anyway stretching between your sets isnt going to make your rest period worthless by making the muscle you're working having to do even more work.
anywho i usually stretch arms legs and back mostly throughout training, then always stretch the body part at the end of the workout.
a guy at my college does weights and for an indian (asian) he is a big dude but he doesnt stretch at all and has absolutly zero flexibility it has to be seen to be belived so stretch always! | 
