Depends how I view my opponent and how he views me.
If we're OOP a good general rules is to 4bet or fold, never calling a 3bet. IP we can do more flatting, but again how we play 3bet pots depends completely on what our opponent's ranges are like: How wide does he 3bet? Is that range polarized or non-polarized? Does he fold to 4bets or does he flat/jam them often? Does he cbet wide in 3bet pots and can he fold well to flop raises? How does he view us, ie will he give us respect for a 4bet or a flop bluff raise?
Basically to keep it simple, I'm only flatting if:
- My hand is good against his range, but not good enough to 4bet/call (that often means hands like AQ, JJ-TT)
- I'm trapping with a monster. This would be against someone who 3bets wide, folds well to 4bets, and/or likes to get very aggro postflop in 3bet pots, ie giving me no respect for a flop raise or barreling very light. Conversely this is someone I'm never set mining against, and am often 4bet bluffing/folding against.
- I have good mining
odds. This is rare and only comes up when we're deepstacked (150bbs? 200+? Zach can probably be more specific here), or when our opponent makes like a minraise. With 100bbs I can safely say just fold your pocket pairs that aren't in the above two categories. You DON'T have odds to call. I guess this changes a bit in FR where 3bet ranges are tighter and implied odds go up, but still it's usually not as good as it seems. Again against someone who I have may have set mining odds against, this is someone who I expect to have a strong range and as such I'll never be flatting to bluff him.
- I have good bluff equity, meaning my opponent gives up well postflop. Going with the last point, this is someone we can't really set mine against. If we expect his range to be wide/weak enough to bluff against, then our implied odds are even worse and we can't mine.