In order to get support for the upcoming Mac App Store, Apple is already working on the OS X 10.6.6 release, which indicates the 10.6.5 release will likely soon be available. According to some reports, it is likely the update will be released within the next few days.
OS X 10.6.5 will be the fifth version of a major release of OS X, and unlike the first few updates, many of the major bugs will likely have already been addressed so updating should be smooth for most people. Despite this, any time an OS update is released, there will be changes to drivers and other core system components that may cause problems for a few users, so unless you absolutely need the update you might consider waiting to allow others to install and test it.
In preparing for the update there are some standard steps you should consider, which include backing up, clearing up current issues, and running a general maintenance routine to prepare your system. In addition, you might consider different possibilities for installing the update instead of just using Software Update. To see these details, read this article about preparing for an upcoming OS X update.
At the very least, before applying any system update be sure you have a full and current system backup available. Use Time Machine to create a restorable backup, or optionally create a clone of your drive before updating, which will provide the added benefit of being able to immediately boot to the prior system setup without the downtime of restoring.