Often I'll see how many players do, and if those that do take it are using their larger stacks aggressively and calculatedly. If they seem very loose, maybe I can double up for free from inducing a shove when I have good hand ;-) Often, other players attempt to do this, only to play too loosely, so be sure to choose your spot and hand. Of course if nothing comes up and average stack size growth/blinds increase is outpacing me then I could just bite the bullet and take the rebuy.
If the large majority (just about everyone) at the table insta-rebuy then I almost certainly will as well just to level the field, as I could get pushed around a bit too often being only half everyone else's stacks, meaning my rebuy after going bust will be a double anyway -therefore I've sunk my original buy in for nothing without ever having adequate stack strength in the first place.
I certainly will if the blinds progress very quickly as I will need to capitalise maximally at every single opportunity and those extra chips may well be the difference between blinding out and getting paid.
Add ons maybe depending on the size of it relative to stacks, remaining # of players but can't think of any particular occasion I haven't taken it. Maybe a few times when I was very large vs average stacks in a satellite and I was already comfortably placed vs # of seats for the taking.
Guess it all really depends on how much you value what's at stake and how rebuy/add on is gonna affect your roll.
Spoken by a true cheapskate :-D