when i feel the finished product will be washed then i wash everything before i cut and piece the project.
if i feel the finished product will never be washed i do not wash the fabric before hand.
i also use tons of hand dyed fabric in my work so by the time the fabric is ready to be used in a product it has been in water numerous times during the dyeing process so any shrinkage has already occurred and the fabric has been treated so that it will never bleed.