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....

A half-yard cut would be enough for the backing for a table-runner.

I hate using up the last of a fabric, so I almost always buy 1 yard at a time. Really don't like fat quarters because using them usually means nothing left of the fabric for stash!

Great information posted. Very helpful

i don't buy FQ's because I think that is an expensive way to go. I buy from 1/2 to two yds.

I have lots of FQs and seldom use them. I prefer yardage. I buy at least a yard......more if I really love it.

If you can afford it I would take 1/2 yards over FQ. There are many cutting instructions for 2 1/2 yard strips or WOW. If you like it for border fabric I would get 1 1/4 yard. If you want it for backing 4 yard for a lap and 6 yard for a twin.

I have mostly FQ and now I am strugling for backing, sashing and border fabrics. Also many patterns I am not able to do the quick cutting methods as I do not have WOW. I think 1/4 yard cuts are no good as you loose a lot of fabric along the edges as it is not straight.

Also most people that are into FQ are big scrappy quilters. Are you? I am but I still need border and backing fabric. I only buy FQ if they are marked down or a good deal.

The minimum I purchase is 1/2 yard. I have pruchased whole bolts if everything is perfect, great fabric ...great price. When I purchase the bolt its becasue I will use it predominetly in the top and use it for backing as well.
As a general personal rule of thumb ...If its a large scale print , I rarely ever purchase under 1 1/2 yards., typically 3 so I can use it in the borders and cut lenghtwise.

I've never bought less than a yard of anything. I like "leftovers". And I ,too, hate it when I've used the last of a fabric.

I usually purchase a little more than I need for a quilt, just in case there is a cutting error. Just recently though, I fell in love with one at JoAnn's that was not on sale and I had a 50% off coupon and got the whole bolt. Then used my 10% off card because I belong to a guild. I intend that fabric to be my next kitchen curtains.