Based on your input, I think I'll leave them in a while longer. Part of my motivation for changing them was to avoid them becoming corroded in place and impossible to remove without damaging the threads in the cylinder heads. I thought I should at least take them out and put some anti-seize on the threads...then I thought, if I have them out, might as well put new ones back in. It sort of goes against the grain to leave them in for so long, but I think I'll wait and reconsider at 60K. The car runs flawlessly now. I'll get a little experience putting plugs in my mother's '02 Camry V6, which is a similar arrangement, but with a larger hood opening and more room around that rear bank. Hers has 75K without a plug change, so I think the service is justified in that case.
As always, thanks for your input. This makes me dread the upcoming 30K maintenance much less.