This is a very interesting topic, and I'm glad you have brought it up (Gillard 2010
).
In all seriousness I have seen all sorts of rules of thumb during my time in GA. The first company I worked for it was all logged off the VDO however these have a tendency to run too slow on the ground so some Pilots added extra to the VDO to compensate.
Another company I worked for added 0.1 to each landing and measured flight time as your wheels off wheels on time. In a bungles queue you could wait an extra 20 mins some days to get airborne and it would not be logged, as you only logged 0.1 for all ground time.
This is against the regs as you are technically not logging all your flight time!
Another company added 0.3 per landing which was for IFR or 0.1 a taxi and 0.2 for a run-up.
In the end I just logged my chocks to chocks time, although this usually equaled about 0.3 a sector which IMO is a pretty good rule of thumb.