If your monitor is anything like mine, it can barely go a couple of months without mysteriously attaining a noticeable coat of fingerprints and dust. While this isn't as hazardous as it is with the CPU, dust on your monitor can contribute to eye fatigue. It also isn't exactly attractive. Before you spend any money on one, make sure the cleaning agent won't harm your monitor. In any case, when cleaning the monitor, wipe, don't rub.
Keyboards can also stand to be cleaned.
Just turn it over and, while holding it, bang on the keyboard bottom and shake it well. All sorts of crumbs should fall out, perhaps even resolving a sticky key problem you might have. And to make the keyboard look as good as new, you can use a cotton swab dipped in a computer-approved cleaning solution to eliminate the grime. And don't forget your mouse. A dirty mouse won't be as responsive to your movements.