I vote with Ned - the register will locate the saw better if it is on the body of the arbor rather than on the end cap. I agree with Graham that the tooth tips of the saw may not be perfectly concentric to its central bore, but it must be better to avoid two small errors adding up to a larger one, where possible.
Though the recess in the end cap doesn't show up well, here's one I made a few months back out of a 2MT blank end arbor to fit my old Dore Westbury mill and cope with some 5/8" bore, 1/16" thick slitting saws:
The bolt head hasn't hampered me so far, but if it does I'll put a thread on a bit of round stock, leaving one end plain to braze into the end cap, and put a couple of flats on the cap for tightening down purposes.
One incidental spin-off of the item is that the 5/8" register just fits in the back of my diestock for small (13/16") dies, so I sometimes get a thread started with the arbor, minus cap, in my tailstock, pressing on the back of the die to get a thread started more square than I can manage without guidance. That takes a bit of juggling to keep the pressure on, so a proper tailstock die-holder is on the round tuit list.
Andy