The advice I hear most often about printing on fabric is to heat set the print before you wash the quilt. That's what we're asked to do for Quilts of Valor, so I figure it works.
I agree with Peckish - fabric softener coats fabric to make it feel softer and prevent static cling, kind of like hair conditioner puts a slight coating on hair to make hair feel softer, comb easier and reduce static cling.
I've never used muslin for printing on fabric, so can't say whether that is the culprit. All my fabrics are pre-washed. I have used freezer paper successfully, but lately my printer is jamming it. My guess is that it either needs cleaning or something in the paper feed is broken/misaligned.
I'll have to try basting spray to fasten the fabric on printer paper, Who knows? It might work!
PS - I've thought about using a ball point pen to make my labels. That never seems to wash out of anything.