Last time I tried Gimp (1 year ago), I searched for an option to rotate an image a few degrees. All I found, was rotate by 90 degrees - like in pre-v5 Photoshop. I went back to Paint.net, which is good enough most the times (sadly it's closed source nowadays and gets little updates) - I open use Paint from Win7 too, for really simple things it's very fast.
That's because image formats are (usually?) constrained to be rectangular. You want to use layer->transform rather than image->transform for what you want (and then probably want to crop so you don't have nothing in the background on the edge).
