IE 9 RTM won't be without bugs You might not be able to upgrade from RC to RTM if there's any changes between the two.
It's as easy as uninstalling the RC and installing the RTM. Nothing fancy. You can use an RC. No one can garantee you that there won't be any bugs, because it's most likely bugs appear. However due to the advanced stage of the software development (RC branch), there shouldn't be any major showstopper bugs. So, theoretically, you can use the RC safely and not experience problems at all. Plus, the IE9 beta was pretty bug free, at least for me, an average user.
of course it's safe, if it isn't safe, then why they released it to public?? RC = release candidate, just like beta, but RC has fewer bugs
I only use RC or Betas for desperate measures when there isn't anything else to choose from at the time and if I really need just that new functionality to get my job done. But when RTM hits I usually ditch them. It's safe, but it's not recommended to have pre-release software and especially not in production envirnoment. Basically what everyone else said here already.