A lot of people use routers to make mortises. I have a mortiser which I usually use although I'm considering making my own multi-router type machine. If you use the router, you'll have rounded mortises, so you'll either have to square the mortises to match the tennons or round the tennons which is easier I think. You may want to do it the other way around though. Clamp the post down and use a plunge router freehand with an aux fence attached to the router.
Do you have a table saw? You set it up once, then run all the boards through. The width should be the same.
You could also make a sled that sets the width of board, then use a flush cut router bit with a bearing to flush-cut all the boards. That would make them identical also.
You should also cut the boards slightly wide, then run the glueup through the jointer and planer again. That should make the sides perfect.
brian