First of all do you mean rebuy or reenter? Rebuy means, you keep your seat and basically just reload or top up, like you would in a cash game. This is mainly possible in tournaments, which also have an add-on, and are therefore called "R+A" (Rebuy + Add-on). These tournaments require a special strategy. But I sense, you are not talking about those but about normal tournaments, so I wont go into that.
Most online tournaments these days offer the option to reenter. Which basically mean, you play the same tournament again, just like someone who register late. You are placed at a random table, and you can only reenter, once you lost all your chips. Whereas you can usually rebuy, when you are below the starting stack, again just like in a cash game.
And since reentering is just the same as late registering, there is no specific strategy for it. If you just started the session and would chose to late register for that particular tournament, then there is no reason, why you should not reenter. But if you would not late register, there is no reason, why you should reenter either. Or in other words, what you are actually asking, is, when you should late register to a tournament.
And this is a very individual question. It depends on your playing style, your goals and what other tournaments are available. If a more attractive tournament will begin in 10 minutes, maybe its better to wait for that. Moreover its usually best to avoid registering to late to PKOs, because as soon as players are knocked out, some of the price money is already distributed. So the first general rule is to pick regular MTTs over PKOs, if you want to late register rather than wait for the next tournament.
The next consideration is, how far the tournament has progressed. If it began 14 minutes ago and have 2 hours of late registration, then its essentially the same as showing up on time. But if the starting stack is only 10-15BB, chances are, you wont be playing more than a hand or two, unless you get lucky. So unless there is an overlay, its usually best to avoid entering a tournament at the last moment. Many good players use something like 30BB as their cut-off level.