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.