I wash everything as it comes into the house. I can't keep track and I don't always have a specific plan for fabric that I buy so it all gets washed. I'm about to make a charm square quilt and I actually laid out the charms on my ironing board, soaked them and pressed them really well and they shrunk about 1/8" each in one direction. Over the size of a quilt that does make a difference, I think. I even wash my quilt batting and woven interfacing. Some people try really hard for that crinkly look, so they don't pre-wash at all. And jelly rolls/charm squares/etc. can't be pre-washed. So you are not doing anything wrong if you choose not to pre-wash.
You won't ruin anything by not pre-washing unless you are using like red or blue next to white. Buy a box of SHOUT color catchers in the detergent/softener aisle. They are little papery fabric sheets that look like dryer sheets and absorb all the excess dye in the washer. Use a couple of those when you wash it the first time and you won't have a ton of bleed. It will be OK

Erin