I use whatever I have at the time..I have made many, many quilts with poly/cotton.......it works just fine, looks fine, washes fine, etc.......I dont wash the fabrics before hand, but I wash every quilt I make, after it is finished... and I press everything as I sew along, always pressing seams,etc.... I haven't ahd any problems using different fabrics, but I do try to keep it all the same in the same quilt, if possible.......like, I don't mix odd things into it, unless the whole thing is ODD things...