Restarting postfix is actually a pretty heavy operation.  It's not necessary when updating a canonical(5) table, because the cleanup(8) daemon is short-lived.  Per the man page:

       Changes to main.cf are picked up automatically, as cleanup(8) processes run for only a limited amount of time. Use the command "postfix reload" to speed up a change.

'reload' is supported, so should be used here, if postfix should be signalled at all.