Pipe, no water at all. I only got some curling and cupping on a few sets and i could file and sand them flat without making them paper thin but i find it *very* annoying
Recently I started to experiment with water (wipe both sides with a soaked sponge) and wrapping in alumina foil and I think it does wonders. For example I had the worst luck with a pair of very very curly maple. First piece bent dry, waist scorched deep (would not bend at lower temp). Second lower temp, I tried harder at bending it, it broke. Soaked it, broke again. Later I experimented foil on them and it worked, bent very fast and easily. Then i tried it again for a pair of Macassar (one normal the other wrapped) this time I didn't feel any improvement but it didn't do any harm either. Today I bent a set of Madagascar rosewood (which I consider a tricky wood, snaps very easily if thick, distorts if thin) and it worked like a charm. The waist bent nicely, it felt very compliant, and there was virtually zero distortion at the end.
If anyone tries this, watch that steam lurking under the foil! Gloves or wooden blocks are kind of mandatory (when dry I had no problems using just the bare hands).
