I am sorry but I cannot agree with Tom Bliss (do I know you?) Many of the very top flight guitarist (no need to name drop) manage to play long sets without the need to tinker with their machine heads. It is interesting to note that only certain people constantly need to do this. Martin Carthy (God bless him) is a great guitarist and we would all like to emulate his music but even he is aware that the constant retuning gets up peoples noses in a big way.
I am afraid the the long, and rather boring lecture on how a guitar gets out of tune cuts no ice and detracts from the joy of making music. GET A LIFE AND GET A GIG.