I'm going to assume that you've read this
, and basically repeat what it says anyway.
Generally speaking, Will, it is best to limit your USE variables to those that you know are important for your system. This isn't a 'hard' rule, but it is wise to follow. More important, however, is ensuring that you don't have conflicting variables.
The system checks the USE variables to determine whether a package being installed should USE other packages. For example, your USE variables will tell the installer whether to install support for KDE or Gnome. If you have neither, or both, the installer might add support for both applications. By specifying KDE or Gnome in your USE variables, you can reduce system bloat.
Of course, some installers are less intelligent than others ... they might bork an install if they find contradictory USE variables.
Finally, by adding 'everything and the kitchen sink,' you are really ruining the whole point of having USE variables.
So ... Jip's rule: add a USE variable only if (a) you know what it does, or (b) you were told to do so by someone who knows what it does AND is familiar with your install. Most of the USE variables are self-explanatory ... so just use your best judgement.