Moe, sinking the pots into the ground for the winter is a great idea. It's even a good idea for the summer, too if you're not keeping your plumeria on a patio for decoration. It insulates the roots from both heat and cold, and will also help retain a little moisture for the plant. Considering how many plants you have, I would think you're going to want to do this, just to save yourself some work. Sink them and then mulch over and around them too. Again helps insulate against heat and cold and moisture loss.
I do this with all sorts of slightly tender plants, and my begonias are all sunk into the ground under my big oak trees because they don't compete well at all with the tree roots. I sink the pot all the way to the rim, then mulch over the rim and the soil in the pot so you can't tell there's a pot there. When they start to look like they're declining, I just pull the pot up again, and re-pot.
