depends on the exercise for me... Standing barbell curls, i do a bit of high reps (8-10 reps), then i go heavy and try to get in around 4-6 reps. With anything like conentration curls or preachur curls, i usually go higher on the reps so i can feel more of a burn. Anything that add's more mass to the overall area of the muscle, i usually do heavy weights with low reps, but for exercises that cause Peak (like in the biceps), or "the cuts", then i do more reps with moderate weight. Just my 2 cents. Fuck i'm runnin outta cash with all these good tips.
Keep humping and keep pumping.