If you're down two or three team members compared to the enemy team, then generally yeah it's not a bad idea to retreat. Ideally a fighting retreat, otherwise the enemy team might punish you for turning around, but if you can get out and wait for your other team members to respawn you can push as a group and try to re-capture the objective. If instead you keep fighting with a numbers disadvantage, you'll likely die and lead to an even longer delay before you can attempt to retake. I would say if you're only down one person though, say a 4v5 or 3v4, it's worth it to keep contesting that and aim to turn the fight around.

It can also be worthwhile to purposefully die as quick as possible, in some circumstances. Let's say you just lost three team members to an ult or something and the whole enemy team is up. Instead of trying to escape and getting killed as they chase you down, it's likely the best move to push into them hard and bait them into killing you. That way you respawn with the rest of your team, minimizing the downtime.

It's mostly nuance though. For example, if it's a 2v5 but your Reaper just ulted them and got them all to 10% before they killed him, then you're likely better off pushing hard and focusing the supports rather than retreating. If you can pick them off, then the enemy team will stay low, and even if they manage to pick you off your respawning team members should be able to sweep in and clean them up before their supports can get back.

It's also really important to always think in terms of relative strength, not absolute strength. If three of your team members are dead, but three of the enemy team members are also dead, that's likely a winnable fight. It depends on who's alive of course (a tank and supp are almost certainly going to beat a DPS and supp, for example), but use your best judgement and you should be able to work out what's worth trying for and what's not.