It's a minimal bit of protection, but I always create a user account named Apple on my Macs, with administrative privileges. It's for use by Apple Techs if they ever help me with technical problems. I them the password and they can use it to log in. I never have them log in under MY user name.
I know that's not going to keep them from snooping through my files if they make an effort, so I still remove files that must be for me or my family only, but it discourages casual snooping. You have to decide how much trouble you are willing go to through for how much "safety".
In general, someone might have a technical problem that has to do with their specific login account, or with specific files in their home folder, so they might think that Apple needs to use their account to solve the problem, but I would never send a Mac in for service for a problem of that type. In fact I'd probably not send a Mac in for any software-only problem. I'd handle it myself, get help online (e.g., at MacRumors), and/or reinstall Mac OS X or apps myself to clean things up.