If you're in NYC, the Levi's store in meatpacking actually does custom denim. I also am obsessed with the 3x1 store in SoHo. You can read about it here: http://www.denimblog.com/2013/08/3x1-store-at-15-mercer-street/
Hopefully a more experienced raw denim expert can chime in as well but I think usually you'll want your true size as they can fit snug and stretch with wear. I would avoid washing your jeans. Here are a couple of other threads that may be helpful as well: this blog post and this thread.