It's up to you.
If you can find the right version of Mac OS X for your "new" iMac, then you can reinstall it as a fresh copy. (If he didn't give you the old OS X discs, which are grey, remember to buy the black OS X retail disc if you decide to reinstall Mac OS X).
Mac OS X doesn't suffer from bloat like Windows, but, if you feel the computer runs slow, then reinstall Mac OS X, or uninstall the apps you don't want/need (there are several third party applications for Mac OS X, such as AppCleaner and Clean App).
Is this an Intel Mac or a Power PC Mac? And what version of OS X are you running?