I almost never prewash (unless it is very red) but I usually do make a test block. If I don't like the block, then I don't want to waste my time and effort on making a whole quilt that I am not going to like. I f I don't like it, it goes in the trash ( which, thankfully, really doesn't happen that often.)
My granddaughter wants a Periwinkle Star variation for her quilt. With the test block, I decided that I did not like all those seams from the 2 triangle pieces, so I added another 1/4 inch and made the background 1 piece instead of 2. It still lays flat, and I like the overall look better.