I'm afraid the only correct answer is "it depends".
If the sound is too bright and edgy, aiming at the glue join will take your head off.
If the sound is compressed and spongy, pointing the mic straight will pick up more transient punch.
If the room sounds really good, backing the mics off a little can be huge, but in a dull or problematic room it will be a disaster.
If the cab or grille has any buzzing or other undesirable artifacts, getting in close might not be so good either.
The mics should go where they sound best for the song and the mix - but that is *not* always going to be the same place, even on the same amp and cab with the same player in the same room.
Luckily, engineers have ears. The greatest invention in audio.
