Smacks forehead, and shouts "D'Oh!!!!"
I've been using arbitrary rotation and just experimenting, adding another couple of fractions over and over and over until it looks right, and then using the Rectangular Marquee tool to select the area to crop, which is a major PITA because you can't drag the borders if you don't get them right. It's such a pain that I'll often jump out and reopen the image in the RAW editor because of it's much more user friendly straighten and crop tools.
In fact I nearly didn't even read this because I thought I knew how to straighten. I did ... the hardest possible way to straighten.
Thanks so much. You've just taken a good couple of minutes per image off my work flow.