The rule I thought of using was 'Best 20' for BAB and each of the save categories.
The Fighter 20 / Wizard 20 / Rogue 20 would have from their first 20 levels: +20BAB, 4 attacks, +12 in each save. The epic advancement stuff would pile on top of that.
Doing it this way, I think it would be fair to state that class granted bonus feats are non-epic until the character has exceeded the normal level limits in a class. Since I'm not really happy with the Epic PrCs that I've seen, I'm not sure what I would do there.
Using these house rules, you are still getting something out of it if after going 20 levels of Wizard, you go 2 levels of Fighter. You get a better Fort save and one more attack per round.