Should I power up a 91% IV Tyranitar.

Is it your first Tyranitar? What is the IV distribution? If you already have one, then you could be a bit pickier and wait for one with 14-15 attack IV.

Do you have extra charge TMs so you could try for Crunch? The current use would be against Lati@s, and Stone Edge does not have a type advantage. Stone Edge is still good though, better than Fire Blast, so if you don't have extra charge TMs Stone Edge will work.

Do you have a good Machamp/Hariyama team and 1~3 other players that you can complete Tyranitar raids with? If yes, then watch for more Tyranitar raids, especially with partly cloudy weather.

1. What is the IV spread? If the attack is 14 or 15 power it up.
2. Do you have any other Tyranitars? If not, power it up.
3. Do you have any reasonable means of getting others quickly? If not, power it up.
4. How much candy do you have? If you have less than 50 then wait.
5. Do you have an immediate use for a powered up Tyranitar? If not, then wait.

Basically, your probably safe to go for it. Otherwise, feel free to add more info for a more detailed answer.

Wow, Gen two as been out for over year and is this is the best you got I would go for it, specially if the attack stat is high. Since you are not asking about evolving a Lavitar i'm assuming this is a TTar that you got in a raid making it a level 20. I would power it up to whatever break point you would need it for but not max it out. You already saved a ton of candy if you got it in a raid instead of having to evolve it. If you ever get a better one, having 2 is not a bad thing.

I would recommend you getting a few more Tyranitaur. They are rather useful.

Set aside maybe 10 of your daily passes for Tyranitaur raids. They are popular so it is not hard to get people to them. Since Ttar is part rock it is easy to get them weather boosted as well. That is a pretty safe and cheap investment.