I'm a fan of the remnants bin at my LQS as well as the one at Joann's (just make sure to read the labels on the fabrics at Joann's to make sure you're getting what you want, they also sell home dec and fashion fabrics which may not be suitable for the quilts you want to make).
I like to make scrappy/modern quilts, so remnants suit me just fine. I also buy yardage in blenders, blacks, whites, some solids and some of Paula Nadelstern's fabrics--all of which work well in my type of quilts.
When I'm buying yardage that is not intended for a specific project I tend to look at it in terms of how versatile it is. If I can see it working in a number of different kinds of project or is in a color way that I need, then it might go home with me.
However, you might want to get some quilts under your belt to see what kinds of things you like to make before committing to building a stash.
Rob