Yeah, but if you have to do coping or whatever it's called, it may not be very simple. I framed out the builder mirror in my bathroom using this technique with a piece of chair rail. I knew I wouldn't be able to get smooth, even cuts that fit together perfectly, so I planned ahead and bought four little wooden medallions (they're usually sold next to moulding pieces in home improvement stores) to cover the imperfections in the joints at the corners. Works like a charm and everyone just assumed they're supposed to be there (and, well, they are). You could try something like that and then you wouldn't have to worry about making perfect cuts.

You can always keep an eye out for old frames at garage sales, antique markets etc. The corners might need a bit of filler if there are a few gaps, but when you are giving them a paint make over afterwards, you can hide a multitude of flaws!