In the old-style ignition systems with mechanical points it would be mandatory. Virtually anything you touch in the distributor will affect timing.
With more modern ignition systems I would usually consider checking the timing to be optional ... but why not? It doesn't take much time, and then you know for sure
that the timing is correct. Many sad stories begin with the words "I just assumed ...."
[This message has been edited by Marvin McInnis (edited 05-18-2006).]