It depends really - it's less of a "when should I try" and more of a "how do I recognize that I can" sort of question - because almost any push situation is going to look or play out slightly differently.
So, I'll run through a few situations:
1. Any time your opponent is on their last point and pops their uber to get out to second. Ideally at least 2 of your team will attempt to go back to last (picking the correct door to go through is the important part because you don't want to run into the non-ubered players on the other team and die instantly lol). If you succeed you win the round, if you fail you've likely pulled back enough of the opposition to hold 2 with respawns (or maybe even their uber if they have no faith in their team).
2. As saam said any time the other team pushes you with players disadvantage is a time to at least look into the back cap (this usually happens when they force your uber with a suicide and then push after). I wouldn't suggest going for this if you're holding last however.
3. Any time your team holds mid and the opposition is pushing mid with a small remainder of players in a transition fight (for example your team pushes 2nd and it goes badly, but there are only 2-3 players left alive on the other team).
My rule of thumb for cap denial is it's worth sacking to deny the forward cap so long as they're stacking it - x5 usually is enough to justify it - the reason why - is if the cap number is very small that means they've sent 2+ players back to stop the back cap (or have some as spawners) and unless you're a dm-lord a regular slow push will suffice for your team (because you're going to die most likely). If you've already got the point half-capped and there are still x5+ on the point sacking your whole team to block is worth it - especially because the point is a target rich environment when your team's dive starts.
You've got a lot of options here though - I wouldn't *always* go for the back cap every time you notice the opposition's flank is empty/weak - a lot of those situations also allow for long flank plays, or for you to simply wait there and give your team another axis of attack when they push back in on a point (basically you're momentarily trapped behind). So I would practice in scrims a variety of those sorts of plays - including simply getting out with your team - because once you even attempt a back cap 1-2 times your opposition is likely to change the way they push lol.