Power up 14/14/14 Tyranitar?

Yes it's still worth maxing out. Tyranitar's uses are currently against Mewtwo, Alakazam and Gengar.

Against Mewtwo and Alakazam, Bite reaches maximum damage before Tyranitar even reaches level 30, so attack IV has next to no difference in raid outcome or damage bonus because the only difference between 14 and 15 attack IV would be a point of damage or two on your charge move.

Against Gengar the difference is only one extra power up to reach maximum Bite damage at level 33 instead of 32.5, but Gengar is one of the easier level 3 bosses to solo that you don't need to reach that breakpoint anyway.

This is how your pokemon looks like on lvl 39.

Absolutely power up the 14/14/14 T-Tar. Remember that it's species and level, then moveset, then IVs. A level 31 T-Tar is better than almost any other mon at level 35. Do it, you won't regret it.