Which is the Better Case

To stack up triggerevents in a trigger, some of which will become derelict as the units that are their subject may disappear, or to 'refresh' the trigger by regularly destroying it and appending only triggerevents for active units?

Depends on the number of trigger events.

For something that requires every unit on the map, refreshing is probably better.

Destroying triggers is evil, but probably acceptable if you're careful about when you destroy them, and only do it infrequently (every few hundred new events probably).

...hum ...as far as i know destroying triggers is (very) evil ...!!!

If you don't know about it, just let 'em build up. Unless your map gets through a rediculously large number of units, it isn't a real problem, and, even then, not a massive one.

If you do periodically destroy the trigger, I'd suggest you first turn it off and only destroy it after a certain delay, since it is believed that DestroyTrigger mainly bugs out when used on triggers that are still running and waiting a bit increases your chance of them no longer doing that (although if your map doesn't use waits that should be a moot point).

I guess the "best" way to do it would be the way I did it in ADamage, simply make it an option; that way you can avoid leaks, however if you start getting weird bugs in your map you can easily turn the trigger destroying off and eliminate the potential cause of the bug.