Hi Sam,
Think of how you will launder and clean the final garment and pre-treat/wash the fabrics accordingly. I pretty much wash all my cottons. Generally on warm, but nicer ones in cool water. Faux silk is generally polyester and won’t need pre-washing, unless you want to get rid of factory odors and manufacturing chemicals. A good press is just right for polys. Rayon WILL shrink, as do cotton and linen, so I recommend any material with rayon content to be pre-washed. (Here’s my post on washing fabric in a machine.) The poly in the rayon-cotton blend helps to keep wrinkles at bay, but you’ll probably still need to press before cutting your pieces. When I sew with true silks I do not pre-treat but for a good steam press.

Here’s a question: do you typically pre-wash fabrics for historical costuming, especially cottons and faux-silks? I’m trying to decide if I should pre-wash a synthetic taffeta for a Civil War ballgown: the bolt label said it’s a rayon-cotton-poly blend, which (for any other garment) I would just throw in the washing machine. What kind of wrinkles can I expect? The prospect of ironing 8 yards of fabric is rather daunting.