When should I let my teammates die?
Never let someone die intentionally. TTK in Overwatch is low -- Heroes will be dying around you regardless, despite your best efforts.
On the other hand, as long as you're constantly healing your allies, you're generating a lot of Ultimate -- so a better question is not, "When should I let my teammates die", but "When should-- HEROES NEVER DIE!".
Personally, If you can land a rez for 2+ heroes, do it -- sitting on your Ultimate won't do anything, and saving it the entire match is the same as not using it. Whether you're on Attack or Defense, you should save it for a concerted push, where the sudden additional allies can swing the battle back in your favor (or secure an already overwhelming advantage as the other team tries to run back).
You should never let your teammates intentionally die just so you can use your ult revive. There's no point of doing this at all, and can only ever put your team further behind. However, if your team is about to be wiped out (say, by a junkrat ult), then you should usually get the heck out of dodge, then shift back in and rez everyone, rather than going down with the ship. In fact ditching teammates which are about to die is a good idea even if you don't have a rez. If it's just you and a Tracer guarding the point, and then 3 offensive heroes come in, then you're better of leaving the tracer to fend for herself and group up with the rest of your team.
So, when should you use your rez then? Well, that's simple! When it matters! Yes, not much of an answer but the idea is that there's no simple answer like "when you'll rez 3+ people". Sure, rezzing 3 people is normally going to be a good idea, but if you just wiped out the entire enemy team, most of the dead allies respawn timer is almost up, and you spawn is right next to the point then you're really just wasting that ult, because by the time the enemy team returns your teammates would have been back even if you didn't use your ult.
So, here's a few questions I'll normally ask myself when I have my Mercy Ult up, and folks are dead:
Which player(s) are dead, and what hero are they playing? If it's that Reinhart whose shield was keeping us alive then I'll happily use my rez to just bring him back, but if it's Symmetra, and her teleporter is already up, then I'd rather save it for when someone else dies.
How many players are dead? If you're going to rez 3 people then unless the entire enemy team is dead you should almost always use that rez.
How well is combat going? If the current battle is going really well for us, and we're going to wipe out the enemy team without me using my rez then I'll usually just save it, since chances are the team will be back before the enemy can pose a threat again anyways.
If I rez, are they likely to just die right away? If it's just you and a Tracer, and three enemies, then using that Ult is just extending the respawn timer on that Tracer. Your better off just letting her respawn naturally and get away from battle.
Does it look like more folks are going to die before the currently dead folks respawn? If there are two dead people, and I see that Junkrat is going to die before I can get to them and heal them then I'll wait a couple seconds so I can get that Junkrat in my ult too.
Are we in overtime? If we're in overtime then I set the bar a bit lower for when I'll use my rez, since it's better to use it and hope it makes a difference, then to keep saving it, only for the match to end with a loss.
How close are to capping the point, or did we just cap it? In either of these cases my bar for using my rez is lower, primarily because if we've got nearly a full team after capping the point then there's a good chance we'll be able to rush to the next point and and get it up to at least the first tick before the enemy team can regroup.
So this is a bit longer than I planned, and there's definitely the more I could list, but the letting your team die just to use your rez is a waste (but don't needlessly go down with the ship by healing a doomed ally), and learning the right time to use your rez comes with experience.
The answer is: when you are going to die from healing them while your ultimate is ready.
Considering that Mercy is the No. 1 target of the enemies' concentrated fire, you'll have to accept that you should be really, really passive when your ultimate is ready. Your ultimate is useless when you're killed off first, after all. There is even an in-game tip making this clear.
Instead, convey them that your ultimate is ready, and ask them kindly to make a bold push through the enemy defense (or a relentless defense against the enemy force, if you're defending). When there is a severe loss on the team, peek into the spot and use your ultimate. This will allow your team to be relentless while making a significant progress, unlike when going relentlessly without your ultimate.