If you're making them with mitered corners, you can make the rabbet first, that way when you assemble the frame, the rabbet will be there, square at the corners. The key is getting the most exact 45 you can on the mitered corners. If there is any warp in your stock, the frame won't go together correctly. There's a reason professional custom frames cost so much.
If you have a table saw that's well tuned, and a really good miter gauge, you can get pretty darn close, if not perfect 45s. Opposite sides of the frame must be identical in length or the frame will not go together square, so you need a stop block to make sure the outside length is just right. Here's the kind of miter gauge you're looking for.
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This is an Incra 1000 miter gauge with the extension fence. It is remarkably accurate, but you must make certain the blade is parallel with the miter slots. I also suggest you use a full kerf blade. The thin kerf can deflect and give you a not quite flat cut. I use a Freud Glue Line blade because it produces a near glass smooth cut. When gluing the miters, apply a coat on each end first, then apply new glue and press it together and clamp it in place with some sort of corner clamp. I'd also consider using glue that dries clear.
Mitered corners are not all that strong, so I cut a 1/8 slot across the corners with a jig, then glue in1/8th inch thick splines. Sanding and finishing produces a very handsome, strong frame.
Are you making very many frames?