Yes I agree with Roughneck. A few years back we had a Rural Metro rep come and speak to us and they said it was not uncommon for them to send out bills of over $80-$100,000 (no that's not a typo) for homes that did not pay the yearly fee and had a fire that they were called to respond to. So where the $400+ bill (depending on size of your home, etc) is a bit steep per year it's better than an $80,000 bill or higher IMO. They do allow you to pay yearly, bi-yearly, or quarterly, so if you need to take them up on that give them a call and they're more than happy to do that for you.
That's one of the downsides of living in unincorporated county land, we aren't really apart of a city when it comes to taxes, etc so we do not have city police, city fire, etc and have to subscribe to a private fire company (this would include ambulatory services as well I believe).
It is your choice you do NOT have to pay, however it's a good idea to.