IMO, it depends what race you're playing as, and the map you're playing on. At some point in a late macro game, it's better to kill your own workers and produce a bigger army, but it depends on many factors. Here are some general suggestions:
Zerg: After you've secured your 3rd, stop major worker production. As you take your 4th base, you should consider cutting worker production entirely.
As you take your 5th or 6th base, consider "killing" some of your workers by making lots of static defenses at your bases, or offensively.
Terran: After you've secured your 3rd, stop major worker production. As you take your 4th base, cut worker production entirely. As long as you have several Orbitals and decent macro, you can spam MULE's and keep up just fine.
As you take your 5th or 6th base, consider killing some of your own workers (or sending them to the enemy base).
Protoss: After you've secured your 3rd, stop major worker production. As you take your 4th base, consider cutting worker production.
Protoss don't have the luxury of spamming MULE's or popping out a ton of workers at once if needed, so I would be more conservative with your workers. Consider killing off some of your own workers after you take your 6th base.
Conclusion: Again, all of this advice depends on the map. If it's a HUGE map where taking 6/7/8 bases is reasonable for late-game macro, then you'll kill your workers off later than if it were a tiny map where the game is over before getting a 4th base.
And of course, you'll have to use your own judgement. Depending on how the game is going, and what sort of unit composition you have, you may want to never kill your own workers. For example, if you have a mineral heavy Zerg army that is very inefficient, but you plan on hurling army after army at your enemy and rebuilding new armies quickly to continually chip away at the enemy, then you'll want to keep your workers and income going.
On the flip side, if you have a very expensive and highly efficient Protoss/Terran army, you'll likely want to kill some workers to make your army bigger and more efficient.