i guess it would depend on how hard the rod [with the rubber retainer ring] pushes into the receiver groove in the master's piston.
sometimes they can be a bear, and in those cases, i leave the rod attached to the pedal.
this way, you can use your leg power to push it into position. this method requires either a helper to guide the rod into position while the master is attached to the firewall, or leave the rod off the pedal and insert into the piston bore after the master is attached to the firewall, then bolt to the pedal.
if the rod can be inserted fully into the master's piston while being bench bled, that is usually the best method.
i have done it both ways.
remember to slightly lube the retainer ring to make it a tad easier to insert the rod into the master.