I guess my answer is, "it depends".
I have a pretty good stash at this point, so I'm not interested in growing my stash unless it's something really special.
Fabric is my #1 favorite souvenir when I travel, though, so I will throw down a lot of money on vacation, buying fabric. Especially if I can find fabrics that are "local" in some sense. I try to find sales but if the perfect "souvenir" fabric is full price, I'll still go for it.
For basics like blenders and solids, I watch for sales and try not to buy anything "in the moment".
If I see a fabric line that I REALLY like, I'll go ahead and pay full price so I can get the colors/patterns I really want. I try to do that rarely, though, and save that for things that I think are really special.
I try not to just buy for the sake of buying...it's hardest when I've walked into a cute LQS somewhere and it's obviously painfully slow in there and the shopkeeper is just dying for some customers. I feel obliged to buy *something*, but I'm trying to get out of that feeling. Sometimes there's just nothing to my taste in the whole store, and I'm getting better about just walking away rather than forcing myself to find something "that will do", just so I'm not leaving empty handed.