You can do it either way, I would use a corner bead, but use the variety that has expanded metal lathe on the edges too. Although corner could be done "freehand", using the trowel as a guide on one side and scraping the stucco over the edge of the trowel (an 18" concrete trowel) let that set, then do the same for the other side. This is done for the first two coats then the finish coat is done on both faces.
If you use a corner bead, you can build up both sides at the time doing the work in three steps instead of five. Make sure you hold the stucco back off the corner bead the appropriate amount for each layer, until the finish layer goes flush with the corner bead
I take it is a sanded finish? You may be able to eliminate a layer, just the scratch coat, then finish coat.