I'm not familiar with specific issues that might be caused with Time Machine as a result of overclocking, but...
Just my 2 cents: in today's age of fast multi-core CPUs, the benefits of overclocking don't really outweigh the additional heat and other stability problems that can occur. If we're talking 3+ years ago (where in some cases you can see up to 50% speed increases), fair enough. But now?
The majority of people that still overclock despite this are bleeding edge gamers trying to squeeze that extra couple frames in. But, reality check: CPUs aren't the big bottlenecks in games anymore. It's the GPUs and as always, RAM.
I have a brother (a PC user) that's big on overclocking EVERYTHING. The funny thing is, he spends more money on cooling and other various tweaks than it would have cost him to just buy the faster CPU in the first place.