You can change the frequency that your hard drive goes to sleep in the energy saver. It's the box that says "Put the hard disks to sleep when possible".
I also use Macpilot, which allows you to set the frequency yourself to a lower or higher number (the default is sleep after 10 minutes).
There has never been any definitive proof (at least I've never seen any) that proves that wear on tear on computer electronics is severely affected by powering up/down/sleep modes etc. etc.
There is a school of thought that says computers should just be left on and never shut down and there are those that believe it doesn't make any difference to their lifespan.
Basically, hot things like hard drives, CPU's and power supplies and the stuff they plug into go bang every once in a while and no one is ever sure why. They just do. Electronics aren't perfect. They are usually built cheaply and are mass-produced.
I would recommend that you carry on doing whatever is most convenient for you and stop worrying about it. At least it sounds like you are energy concious.
If anyone can show me empirical evidence proving me wrong, I'd love to have a link.