I'm trying to write a rock/punk song in the key of A minor and the chorus goes: B5, C5, G5, F5. The B chord in A minor is B diminished, does that mean I have to use the diminished chord or can I use the power chord?
You, as the songwriter, get to decide.
If it stayed in A minor, it would likely be a B and F power chord. But if you like F# better, then go with F#! I'd recommend trying both to see what you prefer, but either one is okay.
B and F would make the vamp fully in A minor. Using an F# would give it a bit of a Dorian feel and would mean you're just moving around using perfect fifths. Both are 100% acceptable in various environments.
Remember, too, that music theory often comes after composition. Thus theory is a way of explaining things, not a set of rules which you can never break.
The '5' chord is essentially a P5: a root, and the '5' 7 semitones above it. Sometimes with the root doubled an octave above.
If you play a dim5 at some point in the song, with all (most of?) the others as just '5' chords, it will probably sound odd. Especially in punk.
Yes, it's worth considering the theoretical side of music when you're playing or composing, but in this case, the theory won't help the way the song sounds. And it'll dampen the flow of the (presumable) guitar playing. Having said that, it may even be a good move in that it will create tension - being a tritone - that the song needs. One person can and will have to decide, unless it ends up on the cutting room floor, which is better.
As Richard and most people here say - theory only tries to explain what has happened, certainly not how something must be done. Hence theory not law!
If you want the 'power chord' sound, I think you'll only be happy with B and F#. (And 'B5' means B and F#, whatever key the piece is based in.
Being 'in A minor' does not restrict your choice of notes or chords. It just tells you what note and chord feels like 'home'. You could sprinkle in e.g. a load of Bb5 chords and, as long as you came back to A minor, you'd still be firmly 'in A minor'. You wouldn't be setting up a new 'home' of Bb major. You'd just be playing a chord that contrasted strongly with your home chord of A minor.
'Theory describes, it does not command'.
You can do what you like, typically power chords are neither minor nor major in themselves as they don't contain a 3rd interval and they are generally used in the same way that you would use single notes in a melody so the overall tonality is more abut the root notes of the chords than anything else.
Often with power chord riffs you have a second guitar, bass or keyboard filling out the chord structure.
In particular it's probably worth having a look at how bands like Korn, Iron Maiden and Metallica structure their songs as they often have quiet melody driven riffs but with fairly complex harmonic structures underpinning how the wehole song works