as a rule of thumb it would depend on if you like scrappy or planned, do you do a lot of gifty smaller things, do you do a lot of swaps...your stash depends on what you plan to do with it..
if you do NOT like scrappy, then you need to buy 3-4 yrds of the prints and matching blenders...which means more fabric in fewer designs..
If you like scrappy, then 1/3 yard is great to allow for shrinkage when you wash it...and yes, you should always wash it before it goes into your stash...trust me....