Steve Elliott
The best method I've seen for small-shop production of tenons combines power and hand tool use. To cut the shoulders, the rails are laid flat and trimmed using a carefully squared-up crosscut table on the table saw and a stop block that references off a point near the center of the rail end. For the short cuts (with the rail standing on its side) the stop block is adjusted to leave just a little extra material, which is then trimmed using a sharp chisel. This takes care of the few thousandths of an inch of misalignment where the different cuts meet.

For the tenon cheeks a tenoning jig on the table saw that references off the same face for both cuts will produce precisely dimensioned tenons in spite of variations in stock thickness.

As for planers, I don't trust them to be precise enough to let the fit of my joint depend on them.