filters are usually a good idea to keep dust out on a normal home or office computer.
i however, do not use any filters or grilles for my fans because I have found that it reduces CFM significantly, and grilles tend to increase the noise level of the fans. however if you dont want to clean your computer, use a filter.
oh I almost forgot-- if your computer is on the floor-- use a filter for sure. mine is on a desk so it picks up less gunk.