There are good answers here already, so I'll just add one word of warning: Make sure you've got the "real tempo" really solid internally. I feel like the number one thing groups disagree on is what the tempo should be (except when there's a click track, and sometimes even then!), and the number one way things go wrong in performance is being faster or slower than the group means to. If you're the one who sets the tempo, e.g. if it starts with your guitar riff, then you can bear the brunt of this blame. Even if you're not, if you're used to a slower or faster tempo you're more likely to drag or to push. So often I see a group learn a new piece and say "let's just play it through under tempo at first," but then that becomes their "first impression" and imprints on the whole group, and they never can stick to the tempo that they originally planned. So for every time you practice either over or under, also play through once at the planned tempo.
After some thought I'll edit to add: We can practice different things. Working with tempo only makes sense with certain technical challenges, mainly left hand. There are some goals that can only be practiced at the performance tempo. One is simply "feeling"—how we plan to do the little tricks of timing, dynamics, and phrasing that turn a boring monotonous riff into a piece of stagecraft. There's usually little benefit to practicing this under or over tempo. Also, you asked about guitar, but for some other instruments like bowed strings or winds, there's the amount of bow or breath you use, and for these, being under tempo can bring new challenges and being over tempo can actually make it easier than the performance tempo.