I wanted to thank IP, once again, for all his help on this thread. I made the switch to the $10/month Airvoice plan a month ago, and it's worked out very well, and saved us $55/month with little change in usage. I thought, especially with Christmas travels, that I might have to add another $10 mid month, but I ended up using only $8, and rolling over the rest for the current month.
I have shut off cellular data for many apps, and I've always had push turned off for most. I know the recommendation is to leave cellular data off unless you need it, but I've been turning it on when I leave the house so I can get iMessages. When I get back home, I'll turn off cellular data and have sometimes been charged a penny or two when I didn't remember connecting to anything. No biggie. A flurry of messages might cost a nickel or so (beats text prices!)
I've mentioned MVNOs to a few people, and most are skeptical, or don't want to "think about it". Okay, then. Me? I'll gladly do a little thinking to save $600/year!