Always auto rebuy, there's not a single time when you shouldn't do that. When you play poker, you should always look for tables that you can beat - tables where you have an edge over your opponents. That means everytime you're not fully stacked, you're letting your opponents get away with loosing less than 100bb, so you're not profiting the way you should when fully stacked.
Ideally if possible, you'd want to be even deeper with a really big whale, you can win even more that way as there are more chances the villain will make a big mistake, whales do not understand stack depth.