I have to admit that I don't use the rate function in myHours, so I'm not really sure.
Do you charge a different rate for different activities or just for different clients? Also, do you breakout your time for your clients by activity or just the total hours for doing the bookkeeping? The reason I ask that is that I've found it's better not to put too much detail in your tasks. I leave it generalized: bookkeeping, administrative, coaching. The reason (and I've found this from they way I work and from others that I've helped set-up the system) if you get too many tasks that means you have to go in and change your timer fairly often and I've found that it doesn't get done. It might work for you...but just something to think about.
ETA: I forgot to explain why I don't use the rate function. I like the hours reports in myHours but the rate/cost reports didn't quite work for me. I didn't find them helpful enough to justify keeping the rates updated for my clients. So, I stopped using it.