I also use Best Press; I buy it on sale at Hancock's or Joann's with coupons. I just love the smell!
Generally I use Best Press when I press yardage before use. Sometimes I prewash, sometimes I don't. If I do, I don't press it before folding. I use starch when I do machine embroidery, sometimes multiple layers to make the batiste or quilting cotton as stiff as a board. That helps a lot with puckers. I don't generally use more Best Press when pressing seams, only if the cotton just needs some extra coaxing.
Be cautious about using starch on items to be stored for a long time without washing. Starch attracts little critters! Best Press and sizing do not.
Pam