If you're taking your laptop in for repair, I do not recommend leaving
any of your files on the hard drive. Back up what you need -- during the repair process, your operating system may be reloaded for any number of reasons, including:
1) Ensuring that any errors/malfunctions are not software-related
2) Ensuring that the hard drive itself is in good, working condition
3) Ensuring that some add-on you've installed is not causing the problem
Even if your repair is strictly hardware-related, like a cracked case, you never know -- your operating system may be reloaded by the tech servicing your computer, and you should ensure that any files that you need are backed up elsewhere. My MacBook went in for a cracked palmrest and the original operating system that my computer shipped with (10.4.something) was reloaded (even though it isn't the original hard drive!). Take every precaution to ensure that something like that will not affect you.
I don't want to come off as rude, but
not backing up your files before sending your computer off for repair would be... uhh...
mucho tonto.