Richard Buscall has a letter to the editor published in the latest Journal of Rheology (subscription required or pay-per-view) that made a salient point or two regarding slip. My favorite quote was regarding the use of roughened or serrated tools in rheometers.
The author once heard somebody accused of neglecting slip say 'Yes, I am aware of the possibility of slip but did not want to compromise the accuracy with which the gap was set and quantified.' Now, one can be sure that that person did not actually mean 'I was prepared to risk the possibility of an error of unknown and therefore arbitrarily large magnitude in order to avoid the uncertainty in the gap setting of, perhaps, 1%'. Except that is what their assertion amounted to."I always love the way the British are able to insult in such an incisive manner.
[*] Even though we all know that it was Brett Farve who did it just because he wanted another day to recover from his injuries.