My DH owns his own business doing this. I agree with PP it can be pricey but it will be a lot cheaper to do it before walls or up.
If you do just do the prewire make sure you have information on where ALL the wires are. I know DH has run into several situations where someone prewired for sound, but then didn't tell the owners where the wires were. This causes problems for the installers.
It is also important to do it before hand because there are some types of installation (foam) that make adding surround sound difficult.
I know that a lot of people like sound bars but they don't sounds as high quality IMO. I agree with PP, they can hide speakers in just about anything anymore.
Also, are you required to use this specific wiring company? If you have the option, look around.
Overall, knowing what I know about my DH's business, I always recommend people at least pre-wire if they want surround sound.
Hope that helps!