These are just general guidelines that i follow:
1) Since most people play rebuys soooo poorly, you should always rebuy every time you bust because your edge is huge
2) Play rebuy tourneys w/in your br; if you have 1k and use a 1% strategy, then don't play a $10r, play a $1r or $2r; maybe a 5r if you play superlow variance and rebuy period is short
3) Never topup or rebuy when you still have chips, generally the blinds are so high that it isn't proportionately worth the value (just like in any mtt, a 2k stack isn't worth double a 1k stack)
4) For 1R1A and 2R1A etc., only addon when the addon is worth more than a starting stack... If it's worth the same in chips than it's worth less in $ because of increased blinds
I'm confident in all of these but #1 is the one im least confident in