Hiyo...
Flashing a new ROM typically changes your Android Device ID (which, nowadays fyi, TiBu actually lets you change back afterwards if you want).. but the Android Device ID has nothing to do with wallet, and don't everyone go flashing ROMs because you think you'll get a new $10, lol. As dmmarck said, we've flashed hundreds of ROMs between us on our Gnex's and our Wallet balance remains the same each time... however, getting a new device that has a new "hard" ID does seem to give people a fresh $10 spot. Happened to dmmarck a few times if I'm not mistaken, hasn't to me yet as I've had my original Gnex since launch.
Now.. if you are activating Wallet on your new device using the same Google/Gmail.. it
should carry your balance over, in theory.. although to be honest I'm still rough on those details so I don't want to say anything I'm not sure about. In any case my Nexus 7 should be here some time today at which point I'll be testing this out for myself and I'll let you know what I find.
Hope that answers some questions/confusion and probably caused a little more...lol