There is every reason to go with the 12v mark, but my experience was not all positive when I tried that, at least when using autogenstart. 11.5v just worked out better. I've seen no noticeable loss in battery strength, but my coach and chassis batteries normally last at least 6 years anyway, and I'm only half way to that point in this coach using the autostart system, so who knows. Certainly battery life is affected some way by any discharge, depending on battery type, but the degree of battery life lost may not be significant unless we're talking deep and numerous discharges.
Additionally I should say I rarely have needed the autogenstart anyway, since I am usually around to start the generator manually before it gets near 12v and I run it back to float charge . If a person does that, I would assume that minimal impact on battery life overall would be the norm. On the other hand, if you use the autogenstart often, and the other control panel settings such as time-of-day, etc., don't factor in to complicate things, then by all means I'd use the 12v setting. Jim's question sounds as if he may not be necessarily referring to the autogenstart system, so the 12v mark he's already using is the answer for him.