Yes, you should clean it out every once in a while, especially if you notice your drains draining slowly. That is why they are there. No one but you can determine how often to do so, because it completely depends on how well your drain system drains and on how much hair and gunk go down the drain. (Some people "shed" much more hair in the shower than others.) Some traps need monthly or even bi-weekly maintainence to prevent problems (say, in a line shared by 4 familys in a multi-family dwelling), others never need to be bothered.
I suggest opening it up to check. If there is no build-up, then you can probably leave it alone until a problem arises. If there is build-up, then clean it out and check it in a month. Then you can get a sense of how quickly stuff accumulates in it.
[This message has been edited by Lawrence (edited July 23, 2001).]