> To write tabs, you'll need to be able to make an educated guess at what's being played.

Knowing the theory certainly makes the process faster because you'll recognize patterns, but you can definitely work through most songs without knowing anything about music theory. Just pick up your guitar, slow the track down and try to reproduce the tones.

Back when I first started playing guitar, my teacher had me transcribe the melody to Rudolph the Red-Nosed Reindeer (from memory). I didn't even know the major scale at that point, but by trial and error I improved my intuition for translating melodies in my head to the fretboard, which is remarkably useful as a guitarist, not only for improvisation, but for composition as well.

That's not to say that knowing music theory isn't helpful in transcribing and in general, but I wouldn't say it's a prerequisite. A lot of my foundation in music theory came from transcribing first and putting things together afterwards.