When you get A 100%IVs pokemon but rate B moves

Heck B moves are not bad, what about C or D? This is common with Tyranitar, you have a 2/3 chance of getting a C or D defensive move set. It sucks. Sometimes the move set is good for offense, then, celebrate!

Depends what the Pokémon is and the specific moves lol...

It absolutely depends on the single case. For example, take a look at Snorlax with Lick-BS. Its a B on defense, but it is still such an annoying defender, that I would power it up in an eyeblink. On the other hand, I got an Espeon with ZH-PB, a B for defense. I will not power it up though, despite of its perfect IV. I want C-PB or FS, but nothing else, because Eevee is so common around here and I have several good IV ones left.

You need to specify more! Which pokemon and what moveset it gets? Species also matters, e.g. dragonite with B movesets and 100% IV might be worth powering up but exeggutors without solar beam aren't worth your stardust even with 100% IV. What lvl/CP it has? Stardust and certain candies are precious, e.g. I would rather power up a lvl 25 89% IV snorlax than a lvl 10 100% snorlax given the movesets are the same cause snorlax candies are rare and stardust are precious to me,

i mac cp most of my 100iv pokemons even thought most of them have worst skills they can:D just skills can sooner or later become stronger or maye they could ve changed in future but iv most likly will never change so for me if you have enough free stardust its good to have them powered up.

you need to be more specific, but if it's a rare 'mon like a dragonite i'd keep it regardless. alternatively, if it's something you can replace or have swaps of, bin it - moves matter more than iv's on both attack and defence.

To prestige level, absolutely.

To defense. Maybe.

As a general rule, moveset is more important than IV for attackers. If you like to collect 100% mons, power it up. If you want best battle results, choose a lower IV on one of the best movesets.

If you are using it as defender, then CP is generally more important than moveset, so favor the one with the highest CP potential (can be quite different from highest IV for Blissey, and to a lesser extent Snorlax).

If you are using it as prestiger, IVs make little difference, and 100% is not optimal. It's generally better to have a 0/15/15. However, you mentioned powering it up, so I'm guessing it is not a prestiger.

Definitely depends on the Mon. I would power up the following at 100% regardless of moveset - Blissey, Dragonite, Snorlax

Other than those, moveset is more important than IV. So a 100% with an iffy moveset is a novelty.