Don't forget that many folks running larger screen resolutions keep their browser windows open at smaller sizes.
This laptop I'm on is 1280x800, but the browser window size I use most often is at about 800x800.
That lets me keep a couple other programs open and visible (in smaller windows) at the same time.
I only maximize my browser window if I HAVE to, and don't much appreciate sites that don't build in flexibility of some sort.
It's a tough call though. Some liquid sites spread out really poorly, and that's probably more annoying than having to maximize the window or scroll sideways.
Maybe consider some sort of user selected "Choose resolution/screen size", where the user chooses between various fixed sizes?