I have never liked 99% uptime guarentee's. In the IT world there are so many things that can go wrong it's not funny, and to offer a 99% uptime is an insult to the customer if you ask me.
My goal is always to have 100% uptime and that's what I tell my customers. Will I guarantee it? It's impossible for anyone to guarentee that your server will stay up, in fact in six years of hosting I have yet to be on a host that's not had some downtime. Sometimes, it's not even something the host can control (network outages).
It's just something that happens, and you have to be able to manage it effectively. When your server goes down the first thing your customers will do is type in your URL and head for the kill.
If your website is hosted on that same server, you end up leaving your customers high and dry to brew.
As a general practice I keep my company website on a server that isn't used very much, in the unfortunate event there is a server outage (which by the way hasn't happened yet

), the customer can have the comfort of mind that it's being worked on.
I have yet to come across a business that makes money by the second, or even by the minute. If you feel like your entire business is going to crash and burn from a few hours of downtime, perhaps you need to restructure a bit or pay to have your server managed by techs 24/7 if your business is
that important.
//End Rant.