Check out www.johnbridge.com for tiling and shower advice. I'm just finishing up a 3'x5' shower and we decided it was too small for a seat. If it gets to where you need a seat, there are such shower seats available at medical supply stores. They're lightweight and obviously not permanent, so when it comes time to sell the house, it will be one less thing for the buyer to worry about.
I put one of these in my mother's shower after the tile was up. You can either do it first, or after...worked well. It's probably easier if you put up sufficient blocking so you can use the wood screws, but the molly bolts work well if your wall thickness is within specs. http://www.innoviscorp.com/better-bench.php Her shower was about the same size as yours, in fact slightly smaller, 32x56.