I also never had a guitar teacher (apart from for 6 months when I was 8 years old) but I've been told by more experienced players that if you rely on external methods for keeping time (like metronomes or tapping your feet) while learning then you might become reliant on them. They say it's better to keep time mentally if you can. I dunno.
I sometimes find myself unconsciously clicking my teeth in time with the beat 
Metronomes are helpful, inspired my name actually... I just don't remember why.
They're definitely helpful. If you ever want to play a piece at a different tempo, it's MUCH harder if you haven't practiced with a metronome (and of course, with it, practiced the piece at different tempos).
John Petrucci recommends practicing with a metronome, so that's good enough for me. In his Rock Discipline video, he outlines how to get started. In one example, he starts at a certain slow tempo playing only 1 note per beat, then progressively increases the number of notes per beat. He does this to illustrate how to develop speed & accuracy at any tempo.
In another example, he demonstrates how to push your limits by increasing the tempo a bit beyond a point where you're stuck ("really kill yourself," he says), then back the tempo back down to a lower speed so that it will seem a little easier. It's pretty crazy to see him push up to 208 bpm.
I think one of the key things Petrucci emphasizes that I'm still not very good at is accenting the beat. Basically, every time the metronome / beat clicks, you pick just a little harder, to help train yourself to keep time, so you know where you're at, especially when playing at higher speeds. I have a feeling that this is one of the important things to develop so that you don't become too dependent on a hearing an external beat.
I'll readily admit that actually watching Petrucci tear it up is one of the things that inspired me to start playing seriously. I still have a looooong way to go myself, but I think I can call my level of experience 'intermediate' without cringing too much.
