If you've done your homework in the kitchen (supplied your body with the right types and amounts of nutrients), and you are able to put out the required intensity during your sets, then you'll get a crazy ass pump no matter what. I get a massive pump no matter what rep range I do. Whether it be 4-6 reps or higher.
Point is, as long as you're doing more in the gym than you did last session for that body part (ie: more reps within a given rep range, or more weight) you are going to stimulate new gains. The pump will automatically be associated with a kick ass set performed.
But there are two different means of getting a pump. One way is associated with
stimulate growth during the set, the other is not. By this I mean, on one hand I could pick up a 10 lb dumbbell and do bicep curls for 50 reps per arm while picking my nose with the other hand. Yes, I will get a pump, but I have most likely done little in the way of stimulating new muscle growth.
On the other hand, I could pick up a 50 pound d-bell and get 6 solid reps using a huge amount of intensity during this set right from the get-go. I'll also experience a pump from this set. NOW, which set do you think actually stimulated growth? Of course, the heavier set I did.
So no, you shouldn't just strive for a pump by going light and doing a zillion reps....but on the other hand, you should be experiencing a damn good pump if you're working intensely with heavy weight and you've done everything right nutrition wise (water intake, carb/protein intake,
creatine supplementation, etc). If you've got your nutrition down pat, you're going to experience a pump right away...even in your warm-up sets.
If you're working the muscle correctly during working sets, your targeted muscle group should be pumped beyond belief. Usually, I feel like my chest, arms, quads...or whatever I am working that day are going to explode through my skin. This is how I know I've got my nutrient intake and training intensity/stimulus where it should be for big gains.
If I don't get that pump while going heavy, then I know something ain't right. Either my diet, water intake, CNS, whatever...and it is time for me to do some thinking on what's going on here.