Always use your ears. If something makes the sound better, it's good. If it makes the sound worse, it's bad. If it has no audible effect then it doesn't matter (unless you can think of other factors, like cramp).
In this instance, you need the ability to leave open strings untouched, so that they sound cleanly.
However if you don't want to pluck that string, this doesn't apply. If you gently touch the string with a spare part of your left hand, then you mute the string, and prevent it from sounding, which is often what you want.
Indeed, if you can't mute that string with your left hand (due to the other stuff the left hand is doing), then you often need to mute the string with your right hand instead.
Other times, if you don't mute, even though you don't pluck the string, it will vibrate due to resonance. This is audible. For a clear demonstration, leave all the guitar strings untouched, pluck the bottom E string loudly, then immediately mute just the bottom E. The other strings continue to sound, with the resonant frequencies.
Sometimes this is musically desirable, and sometimes it is not. So you have to make the choice from moment to moment.
The decision is affected by what kind of guitar you are playing.
The notes on classical guitars don't sustain for long, and you're less likely to accidentally hit a string with your right hand. Muting is necessary, but is a small part of all the other things you're doing.
On an electric guitar through a distorted amp, the tiniest touch of a string triggers a loud sound -- so controlling that through muting becomes a major factor of playing a distorted electric guitar.