The best way to run a system is not with an iron fist. That may be how you run the military, but UNIX was not designed for such discipline. It makes sense to lay down a simple and flexible set of guidelines for users---but remember, the fewer rules you have, the less chance there is of breaking them. Even if your rules for using the system are perfectly reasonable and clear, users will always at times break these rules without intending to. This is especially true in the case of new UNIX users, who are just learning the ropes of the system. It's not patently obvious, for example, that you shouldn't download a gigabyte of files and mail them to everyone on the system. Users need help understanding the rules, and why they are there.
If you do specify usage guidelines for your system, make sure that the reason behind a particular guideline is made clear. If you don't, then users will find all sorts of creative ways to get around the rule, and not know that they are in fact breaking it.