While MacOSX is in general more secure than most versions of Windows, it does not yet have a level of security equal to specialized secure versions of UNIX. OSX is about as secure as it gets for the consumer market, though. Just be aware of the fact that your system will not be 100% secure no matter what you do. Getting a little paranoid and building in extra layers of security isn't a bad idea. Make sure to check the clients for unauthorised apps and activity often.
I have administered both Windows and Mac open labs. I have often been surprised at the level of skill and determination demonstrated by 1% of users. In my case, protection of the data stored on most of the computers was not critical, which made things much easier. Critical data was placed on a fileserver in a physically secured location. Nevertheless, unauthorised usage was a problem. We had a number of talented hackers at the school I was working at that liked to cause mischief, especially with the OS7-OS9 computers. On the OS8 machines I usually installed a clean disk image daily since they were the most popular targets. As soon as OSX came along things got easier after the transition, but still not perfect.