HI dw, and welcome to the Grumble.
I see by your profile that you are an "explorer"?
I am going to throw out an educated guess and say that you are not a framer by the question you asked. That's OK.
A 90º angle is a 90º angle no matter what you cut it with. Changing the cut will only change the angle of your corner and throw something off. I suspect that you bought or were given some "moulding" to build your mirror frame from and it isn't lining up correctly after you cut the corners.
What kind of moulding are you using?
Is there any markings on the back of the moulding to identify it?
What did you use to cut the corners?
How are you trying to assemble the corners, vise, little corner clamps, your hands?
Have you ensured that all your cutting/joining equipment is calibrated correctly?
Supply some more information about your project and one of us will be able to come up with some answers to help you along.
Framerguy