Depends on how many people are in the game, and how many start off with a rebuy.
Start slow. See how things go. For me, I always seem to do OK when I do that. It may be that you play it right, and don't need to rebuy. Maybe even so good that you don't need the add-on (they tend to go hand in hand).
If I rebuy immediately, I tend to get into that 'rebuy fever' and throw sanity to the wind. I do not remember that ever working well for me. It might have, and other things about that tourney pushed the trival rebuys from my memory.
There is a variation on the rebuy/add-on games where you are limited to 1 rebuy. The argument there will be that you don't want to miss an opportunity to double up to 4 buy-ins (or close) early on. Rebuttal is that you only get one major mistake.
If you join rebuy/add-ons late then the trick is to rebuy immediately, and do the rebuy, so that at the rebuy cutoff you are sitting at 3 buy-ins, which is seldom a bad place to be sitting at the 60-90 minute mark. I base that on the notion that in most tourneys (large) at the 60 minute break, about 1/2 the field is gone, and so the average stack is about 2x the buy-in. Last time I did a R/A was before BF, and both the major sites had extended the rebuy period to 90 minutes, which roughly implies 3 buyins average stack size.