I have a sure-fire way to tell when to clean my LCD screens:
When I want to make fun of someone (some web page) for improper/misplaced punctuation (a period or a comma) and I discover it is some dust or dirt or splashed liquid on MY screen that LOOKS like punctuation.
The worst time to have a dirty screen is when I'm using php and trying to find an error in a script. (Where an extra period can totally break a script.)
Y'all realize, of course, that this is simply the present millennium's equivalent of cleaning your glasses.
(Where an extra period can totally break a script.)
Huh. With me it's generally a missing semicolon. Can't remember the exact permutation but there's one place where the semicolon is optional in js but mandatory in php. There's nothing like firing up MAMP and coming face to face with a perfectly blank screen.