"Second chance" tournaments are just completely normal tournaments
Whenever this question comes up, people always massively mess up the different tournament formats, which excist. The most common online tournaments today allow one or more "second chance" reenter or rebuy the only difference being, that if its rebuy, then you keep your seat, but if its reenter, you are usually placed at a new table.
But in both situations its just like playing the same tournament one more time, and there are no strategic adjustments to be made. Basically all, you need to ask yourself is, if you just opened the poker client and wanted to start playing, is this the tournament, you would choose? If yes then go ahead and press that rebuy / reenter button, if no then dont.
Typically if I bust very early due to some kind of cooler situation like my KK running into AA, yes then I rebuy or reenter. Why would I not? If on the other hand the tournament has been running for 1,5 hours, and a starting stack is now only 20BB, then I probably dont, because I think, some of my edge comes from being there from the beginning. Also other tournaments will have started, so unless its something very special like the Sunday Million, which only runs once a week, I might as well play something else.
R+A tournaments are something completely different
Then however there are the R+A tournaments, which are something completely different. Its basically like a hybrid between a tournament and a cash game for the first typically 60-90 minutes, and usually allows unlimited rebuys. And unlike the "second chance" rebuys, you can rebuy, as soon as your stack is below a starting stack. This in itself is not a +EV move, but I might still do it, if my stack is about to become shorter than 10BB, or if there are huge fish at the table, who have me covered.
At the end of the late registration period players are then allowed to add-on, and this is, where the R+A really differ from other tournaments, because those add-on chips are cheaper than chips purchased during the rebuy phase. On
pokerstars they are 33% cheaper, and on
888 Poker its even 50% in most of their R+A´s.
This mean, you basically always have to purchase the addon chips, because even if you are the chip leader, there is no scenario, where the ICM value of additional chips is less than half the ICM value of an average chip in the tournament. So on
888 Poker I have enabled auto add-on, so that I dont ever miss this opportunity due to being busy with other tables etc.
Purchasing that add-on, while some of your opponents dont, is probably the easiest edge, you can find anywhere in
online poker. This also mean, its pretty bad to not make it to the add-on period, so usually you should be prepared to rebuy if needed.
The only times, I dont rebuy in a R+A is, if I am tilted after busting, the table is very tough, I want to end my session earlier than planned, or I want to reduce my table count. If for instance I am running deep in a 22$ MTT, then focusing on playing well there is more important to my EV than staying in a 2,2$ R+A, which I also entered.
It can also be a reasonable strategy to register in the very last moment in an R+A to reduce the risk, that rebuys are going to be needed. In that way you will start the shoot-out phase as a below average stack, but you will have paid less for your chips than what, they are worth.