This is from the Jobo quarterly Issue #3, (http://www.jobo-usa.com/jq/jq9403.htm). It seems to conclude that this topic is pointless to argue.
>>"JOBOís recommendation of a 5 minute prewet of B&W film is probably one of the most disputed suggestions we have made. It causes many people to choose up sides. Certainly many people have chosen to use the prewet, and are more than happy with the continued excellent results they obtain. They have made the other necessary adjustments, and their process is clean and consistent. The original suggestion came from Germany, with the statement that the H&D Curves were closer to the manufacturer's curves when we used a prewet with rotary.
Several technical writers/lab technicians in Germany disagreed with the idea of prewet. Using a JOBO processor they produced a list of films and developers, with processing times, NOT using prewet. We have tried some of these combinations. Every one which we tried performed beautifully. The chart shows what they produced.
More information on the effect of prewet comes to us in an article in the May/June 1994 issue of Darkroom & Creative Camera Techniques. Phil Davis (Beyond the Zone System) has an article entitled "How Prewetting Film Can Affect the Image". While Phil is not dealing with rotary processing in his article, his findings apply. To quote his conclusions, "In most instances, prewetting will influence image formation to some measurable degree. The probable result will be some increase in image density and contrast, and occasionally a slight boost in film speed. It is also apparent that all films are not affected the same way, and an individual film's response to prewetting may not be the same in all developers."
Of course JOBO has a track record regarding the use of the five minute prewet, but it is apparent that this type of information was not available when we established the blanket 5 minute approach. The best advice that can be given is to echo Phil Davis and Paul Schranz. They were writing 7 years apart, but have said the same thing. Iíll quote Phil Davis, "...it is a matter for the individual photographer to decide, so I leave it up to you to determine how prewetting affects the materials of your choice." What more can we say?"<<
There you go... clear as mud. Do whatever the heck you want.