how often should one re-evaluate their grip?

How often should one evaluate their grip? VERY VERY often. Should you think about it on every shot? You shouldn't think about anything in an ideal world. If you get to the point where you can put your hands on the club correctly every time without thinking too much about it, you will be a long way to being more consistent.

Even the best players get "grip creep" where they let their perfect grip slip into bad habits.

1. Know what your perfect grip looks like and feels like.

2. Make it a habit.

3. Review often to make shure your grip is still perfect.

One of my recurring themes is to suggest new players (like myself) taking lessons on a fairly regular basis.  How regular is, of course, open to debate, but about the time you are feeling like your grip may need adjustment it is probably time to pony up for another.  The grip should be about the first thing a pro checks at the start of a lesson.

I don't agonize over it or anything, but I am very conscious about taking my grip at the beginning of each shot.  Depending on the shot I am thinking of, I may make the grip a bit stronger or weaker, choke down a bit, etc.  That helps me to avoid slipping into bad habits to some degree I think, and I guess you could say I "evaluate" my grip each time. To me, the grip is mighty important.