It's no big deal if you don't mind having a few partitions on your system. Personally I have a large partition for my /home directory, and smaller partitions for testing out new releases, distributions and such. If one of the releases is/becomes unstable, you can just select your old reliable release in the boot loader and get work done on that. It has worked out really nicely for me.
The only issue would be if you run a newer version of a program that updates it's config file (or something similar) format, which ruins things when you boot back into an older partition running a prior version of the program.
