I have not done a helicopter tour, just bus.
But if I were you, I would book in advance.
1) It could be a time commitment of anywhere from 3 hours to a full day, so having it blocked off in advance will help you plan the rest of the trip.
2) My experience with the bus tours at least is with research and googling you can get better deals that you'll be able to buy at a random booth in Vegas. And just as importantly, you will be able to research reviews of the companies you are looking at. How long do they stop where? How big is that snack? Etc.
3) You might want to research and decide what time of day makes sense to do it, and have the flexibility to book that.
I wouldn't book way in advance, but if you are sure you want to do it, I'd have it booked before I got there.