Power up all 15/15/14 ?

Assuming we're talking about a relevant pokemon, and not a Sudowoodoo, then yes, absolutely yes.

I only max generalists (Dnite and Mewtwo) and perfects.

I max the ones that I plan to use for battles/raids only. I have some good 98s and 100s, including Lapras and Arcanine and Gyardos, that I haven’t touched because I already have powered up mons to cover these roles and don’t want to waste dust on my 8th Gyardos.

Assuming a useful Pokémon?

YES. YES YES YES.

YES.

(Well, level 35 is where I currently stop, but when I've got most of my useful Pokémon at Level 35, I'll start bumping them up higher.)

Anything meta relevant enough that I have a use for or a pokemon that I like enough. Which makes a rather short list at the moment since I have a lot of maxed pokemon already. 98% mons I would personally max right now:
Dragonite (would be third maxed)
Machamp (would be second maxed)
Gardevoir (trophy)
Gengar (better clear times for Alakazam raids)
Alakazam (trophy)
Mewtwo

I have a 15/15/12 Alakazam which is the only thing I have maxed right now, at level 37 (im 35)

If it's meta relevant or best of its type, yes I would whenever I have the stardust and candy for it. Maxing out the rest is only reserved for the 100% Pokemon should I ever have too much stardust lying around (can you ever?).

My only maxed pokemon has this exact stats; Raikou

I usually only care for 15 attack mostly. Defense is second by a hair over stamina but will pick stamina over defense in some cases. Some pokemons a 97 and 100iv is no difference except cp. Take a tyranitar for example. A 100iv and 15 15 14 ads has same attack defense and hp at level 40. Only difference is 9 cp. I maxed out a 88 iv 15/10/15 geodude I hatched a long time ago to level 40. Also maxed out a 86iv mewtwo. I also maxed out a 86iv dragonite. I think those are my lowest iv Pokemon I have maxed at level 40. Mewtwo is 14 attack and dragonite is 15 attack. As long as i can hit the last breakpoint with 15 attack at level 40 against a certain raid boss is mostly all that matters. Some specific cases I would be very choosy like gengar. I have 2 level 40 gengar 97 iv 15/15/14 ads. If gastly has 14 defense, 14 attack or 12 stamina I would transfer immediately. So a 14/15/15, 15/14/15, 15/15/12 would be worthless gastly to me. Only 15/15/13 plus would do since I can take 5 confusions from alakazam or 10 psycho cut and one dodge of future sight.

My lugia and groudon have 15-15-14 and maxed out.
Edit: and i have a 15-15-14 kyogre which will go to 40 soon.