Sounds like a good gig for you to be so aware of not wanting to sabotage it, enjoy!
I'm torn on volunteering. I worked this year and it didn't allow me to enjoy the festival in the same way as previous years, plus I was way more wiped out post-festival from burning the candle at both ends (16 hours sleep in 5 days and 32 hours of working Thurs-Sun).
My day to day work means it's very difficult for me to be at Glastonbury anyway, so ideally I would always arrive Thursday and leave Sunday evening, but this year working meant I had to be on site Wednesday to Monday. Working/volunteering manes you have to be there for a longer period, so the only way I should do it is by getting a ticket. Being there for less time matters less if all your time is spent doing what you want to do.
But if I don't get a ticket, I'm sure I'll be scrambling for work/volunteer positions.