I like that approach. I was going to treat myself to a ProVari when I hit 6 month's smoke free. I moved that up and got my 1st one at 2 months and my second one at 3 months. I punished myself at 6 month's and didn't buy another one.
I was up to 10 510, slim batts in the beginning. I was going through about 1 batt/hr and the constant recharging with the original 2 from the starter kit was driving me nuts.
I rotate 4 batts per day between 2 ProVari's and have 2 batts for spares. I too have them numbered in case / when they start to degrade I can tell them apart.
I think this is the best thing to do is treat yourself