My technique for recording guitars thus far has been this: one mic on the speaker (the actual speaker inside the amplifier) and one mic at the other end of the room, sometimes suspended up near the ceiling.
This way, you get your guitar sound, but you can mix that with the natural reverb of the room you're recording in.
As for mic placement on the amp itself, you can pivot it 45° to the left or right, but I usually just point it directly at the speaker. Be sure to adjust your gain here so you're not clipping.