Dave, as a former competetive(in my mind anyway) road runner I lived (and often died) by my watch. The reason I started measuring was that I was so tired of races where I knew by the splits that there was something obviously wrong with the course and I couldn't depend on the distance being accurate.
There was a local 5K race some years ago where I took my split at the 1K and then at the mile and was surprised to find that I had just run that 600+ meters in an amazing time. I got myself in hot water with a few people by measuring it later and letting a some know that the 5K was really only about 4850 meters. I should have just kept that to myself.
I have a fairly good relationship with the local people I measure for and they have come to expect the kilometer location documents I produce. I'm not sure what I would do if I was to travel outside of my home town to measure. I suppose I would have to overhaul my routine to make it fit with the time available.