Because Skeeter Pee doesn't typically age before bottling, I don't top up. In fact, the procedure I use is:
1) rack from primary to 6 gallon secondary.
2) let it finish fermenting (the ferment will create CO2 and push the O2 out the airlock)
3) do the stabilizing, degassing, fining, while trying to minimize the time it's exposed
4) rack when clear to another 6 gallon carboy and add sugar to sweeten (the sugar will take up some of the headspace, so having it can be beneficial)
5) wait a week or so to make sure ferment doesn't restart and bottle