We have agreed that if it can be done in the darkroom, it's allowed. An example of this is Ansel Adams's Moonrise, Hernandez, New Mexico, 1941. Over years he reprinted this photo, continually manipulating contrast and exposure until he reached the classic version we know today. Certainly manipulated, and certainly allowed under our rules.
http://www.hcc.commnet.edu/artmuseum/anseladams/details/moonrise.htmlThe exception is extreme manipulation unless there is a specific contest for that. We haven't yet discussed the inclusion of HDR because not many of us do it yet. I suspect that would be considered under special contest. The object here is to make good photographs. So sharpening, increasing contrast or saturation and such is part of that.