I like mic-ing one of the bottom speakers, at the point where the cabinet is at it's maximum depth. There's a stain on my grillecloth- I've always used that as the spot for mic placement both recording and live.
It's not much of a big deal if your cab is not rear ported. For the most part, a mic on your cabinet is going to pick up what's coming directly out of the speaker. That's why you'll usually have a 57 or a 58 right on the grillecloth, and not miced 3 feet out, or angled off a wall, or whatnot.
Say you have two completely different speakers in your cab, which do you pick to mic?
For example, I have a mix of V30's and G12T's...if I have to pick one, I go with the G12T because I prefer the punchiness of the bass over the thick mids of the V30. But it's a tough choice...there is a reason why I use a mix of both kinds.
Which ever one sounds best run through the PA. At the point you're already getting your stage volume from the cabinet as a whole- you decide what speaker the audience is going to hear. Personally, I would go with whatever has a upper mid and treble presence- since the bottom is going to be filled up with mostly drums and some bass.
a little off topic...but the last time my band was recording, i marked an X on my grillcloth with one of those silver sharpie markers, so i'd know where the mic was placed in case it got bumped or anything like that. one of the following days after my tracking was done, we had a show down in detroit, and of course that X was still there on my cabinet. and can you guess exactly where the sound guy put the mic?
whenever you pick where you want the mic to be, or which speaker you want it to be on, then the sound guy will understand if it's visually marked.
Say you have two completely different speakers in your cab, which do you pick to mic?
For example, I have a mix of V30's and G12T's...if I have to pick one, I go with the G12T because I prefer the punchiness of the bass over the thick mids of the V30. But it's a tough choice...there is a reason why I use a mix of both kinds.