I pre-washed my Tmax 100 MF 120 for 30 seconds with water, when I dumped the wash into a bucket it was very dark/purple. I developed as normal and when the developer was added to that bucket the purple water was cleared (or was it when I added the fix?), either way there was undeveloped material in that pre-wash water. My negs were a little flat. I'm wondering if pre-wash with roll film is not required. I always prewash my 4x5 TriX 320 with great results.
Is Prewash recommended with sheet or roll film? Would it have mattered, i.e. does the developer "stick" the image to the base, or can that undeveloped latent image be washed away or degraded in 30 seconds of water prewash?
As Christoph already said: Do it or don't!
However if you do it, do it long enough, or you risk uneven development, which is the exact problem presoaking is trying to prevent. Presoaking has to be long enough for the water to evenly soak into the emulsion. In my tests, this was between 3-4 minutes with Kodak and Ilford films. Consequently, don't presoak for less than 5 minutes, or you will risk water spots.
The other problem is, presoaking does extend the required development time. The developer has a harder time to get to the silver in a swollen emulsion, hence, slightly flat negatives can be the results, unless this was compensated for through an extended development time (10% rough guess).
my opinion:
Presoak or not. I think it's a waste of time but doesn't hurt. But, whatever you do, test your films either way, and never switch, because film development is visibly affected by presoak.