In a musical theatre piece is it wrong to have lots of parallel octaves in the voices?
The rules about parallal octaves only apply when writing Bach chorale-type harmony where the aim is rich harmony with no one part "sticking out" disproportionately. Because this is often the first type of harmony we are taught to write, we can fall into the trap of thinking it's the ONLY way of doing it!
Orchestration is all about doubling lines, often in different octaves.
For instance, there's the "Glenn Miller" voicing - clarinet and tenor sax double the tune an octave apart, the other reeds fill in harmonies inside that. This works for voices as well. (Note, though that the aim is to emphasize the melody, and it works. Don't double something that you DON'T want emphasized.)
It can also be very effective to have just two musical lines, either in homophony or in counterpoint, with a mix of high and low voices on each.
I could find many more examples. But yes, octave doubling is fine. Just be aware of what it will sound like, and make sure you WANT that effect.
The only time you shouldn't have parallel octaves is when you are voice leading and want two parts to be completely independent. The reason why you wouldn't use it is that it makes two voices that should be independent sound as one.
It's used very, very frequently as doubling a line by octave is very effective at making it stand out. For example in Day Tripper the guitar and the bass play the same riff so everything in that riff is considered parallel octaves, but it sounds just fine and actually puts more emphasis on the riff. Pretty much any time you play the same chord shape on guitar, like moving from an A shaped D chord to an A shaped E chord or from power chord to power chord, the result is every note has a parallel octave. This is still used all the time though and does create a distinct effect when it is used.