I do both. I have a good stash and plenty of scraps to keep me busy for some time, but if I am in a store and a fabric speaks to my heart, I will buy a couple of yards because I know I will use it. This is especially true if it is on sale. I seldom by a $12 a yard fabric that speaks to me because my debit card is yelling much louder, "What are you thinking?". On the other hand if that $12 a yard fabric is the perfect piece for a work in progress, then I will measure twice and buy only what is needed and round the yardage up to the next quarter yard so I have some scraps for my scrap bags. (This doesn't happen often because I seldom go into shops that sell those high priced fabrics.)
Interesting question. I had not given much thought to my shopping "philosophy" until now.