I'm completely hit and miss with my prep! Sometimes I prewash everything beforehand. Sometimes I prewash most fabrics but forget to prewash others. Sometimes I don't even bother to iron the fabric to get the crease lines out!
I guess it all depends on how excited I am to get on with the project! Usually when I forget to prewash I will realise when I make the last cut of all the cutting out, and then curse myself!
I've only had one major issue when the completed quilt is washed and that was from a black border that I hadn't prewashed. I've since invested in boxes of colour catchers and haven't had any other problems!
I've not had much shrinkage in any of my quilts, apart from one that I meticulously prewashed every bit! Although I think that was probably down to the batting (even though I prewashed that too!)
Fingers crossed I don't have any major issues in the future!