I do a rebuy when the blinds do not exceed 50/100 chips, this is so that when you re-enter the tournament, the stack remains more or less game-like.
I never make more than one rebay, because two rebays are very difficult to win back, it is necessary to get at least to the final table.
In general, I try to play without rebuys and reentries.
I usually don't re-buy in a tournament, I really don't like it, but if I must confess that I have rebuyed in some tournaments, once it was by mistake and the other was my decision (bad decision), I was blinded for the moment and the way I lost, I know you should take your bankroll into account when playing this type of tournament
If the tournament has an add-on, my first goal is to make it to the add-on break and purchase the add-on, and in principle I will rebuy as many times as needed to acheive that goal. I do occationally jump out before the add-on break though, if I am fed up with the tournament or want to focus on other tournaments or end my session early.
In other MTTs its usually called reenter, and on most poker sites you will start again on a different table. So its just the same as registering late. I dont have any particular rules for this, so its very situational. If I get coolered within the first 2-3 blind levels and lose my stack, I will usually reenter, especially in MTTs without bounties.
I might not do it though, if I was taking a "shot" at a slightly higher limit than normal, or if I want to focus on other tournaments. On 888 Poker you get to keep your seat, and then I might also not reenter (they call it rebuy), if I dont like my table or seat. Late in the tournament, like with stacks of 25BB or less, I will usually only reenter, if it seems like, there might be an overlay. If that is not the case, I will usually prefer to play another tournament, where I can start with deeper stacks.
If its a great tournament that rappens 1 time / day
The blinds are not too high compared to the starting chips
There is no other interesting MTTs at the same time (with the buy ins that i play)
So i'll rebuy
Its a lot of factor.
For exemple: I like the HU 100%PKO MTTs Pokerstars, i play from 3.3 to 11USD buy in on those tournaments. If its the last of the day and the initial stack still playable, so ofcourse i'll rebuy
(And if i'm not rebuyin to play like an idiot. Sometimes people gets a badbeat, rebuy, and start playing like an idiot to get the chips back as soon as possible)