I only upgraded from Mountain Lion to Mavericks so I can't add anything to the above regarding old applications. I'm fairly religious about keeping my apps up-to-date (apart from Adobe!) and I upgrade the OS whenever there's a new version available so by and large my experience has been problem-free (momentary confusion about where they've relocated things nonwithstanding).
What I can suggest is that, after upgrading your apps, that you follow this procedure for your upgrade:
1. Make a complete backup of your entire boot hard drive. You can do this with Time Machine, you can do a bootable clone with a number of third-party programs -- whatever your preference, just make sure it's done. Disconnect the drive once this is done (you'll see why shortly). Also make sure you have plenty of HD space available (25GB or so, at least).
2. Run the version of
OnyX (free, scroll down to find the right link to the version for 10.6.8) that is appropriate for the OS you have NOW, before the upgrade. Just use the default "automatic" settings.
3. Download the installer for Mavericks from the Mac App Store. Optionally, you can choose to make a bootable USB thumb drive copy of the installer for possible future use using
Discmaker X.
4. Following the install of Mavericks, run Software Update to catch up on any recently-released updates. Run it at least one additional time to make sure you got them all.
5. Download the newer version of OnyX for your new OS version, and toss the old one.
6. If you used Time Machine to make your backup, don't plug the backup drive back in for a few days. Mavericks is a big jump up from Snow Leopard, and a few things will be confusing (why is the scroll "backwards"? for example) at first. Give yourself a few days,
read a guide to Mavericks, get used to the changes. Once you are sure you're happy with performance and have customized to your liking, THEN start making fresh backups, overwriting the Snow Leopard install.