I always wash fabric intended for garment making; never for P&Q.
As far as pressing is concerned, as
@BrendaP says - it's horses for courses. I agree with all her statements for dressmaking. For P&Q I usually press my seams open - which I know is controversial BUT I do use a very tiny (1.5) stitch for piecing so the chance of the wadding finding its way through, or the seam being weak, is almost zero.
I find I get a much better finish by pressing the seams open, and if it's good enough for Leah Day, it's good enough for me.
(I have been known to press seams to one side if making very, very simple projects, as it's simply quicker.)
I don't use very much starch although I do have a couple of spray bottles of Best Press on my shelves. I only use it if the P&Q fabric is being particularly stubborn. I've never used it for dressmaking.