An add-on reduce the value of chips, so its kind of like another layer of ICM, which you need to consider. Lets say for instance, its a 5$ R+A, initial chips are 3.000, add-on chips are 6.000. If you have starting chips and lose a flip, it will cost you 5$ to rebuy. However if you win a flip, your 6.000 chips will not be worth 10$ after the add-on, because other chips were now purchased paying only 5$ for those same 6.000 chips.
For this reason why want to be more conservative risking your stack before the add-on, than you would in a normal tournament. You want to avoid a situation, where you rebuy multible times and perhaps even end giving up, before you make it to the add-on. Its just the same as buying clothes or groceries. You want to buy as much as possible, when the store has sale, and as little as possible paying full price.
If the field is very soft, it can still make sense to be there from the beginning and take advantage of maniacs, who dust off stack after stack. But in tougher fields the best strategy is actually extremely simple. You just register in the last minute, fold a hand or two, and then purchase the add-on chips.
If a tournament only has rebuys but no add-ons, this does not tend to matter much. You can however use it to do a little bit of table selecting by rebuying, if you are on a good table, but firing only one bullet, if you are on a bad table. If for instance you have a huge fish on your right, who just won all your chips, you should almost certainly rebuy, because that is an extremely +EV situation to be in. Cash game players call it "the jesus seat".
Reentry, where you start again at a different table, is just the same as late registration, and there are no adjustments to be made. Just because you are allowed to play the tournament multible times does not justify a more aggressive strategy or anything, since you also pay multible times. So you might as well just spend your money playing more tournaments.