For example:

7 7 6 6
- - - -
9 9 8 8

What if there is no "X" in the tablature. How would you play that?
You could still mute it if you want, doesn't realllly matter. That or you could use your thumb and index finger to finger pick it.
I usually just mute sections like that lightly with one of my fingers.
Play it like there's an X there anyways, it's not like you can hear it when you play, especially if it's a rock or metal song.