If you want None then set the MosaicDataset Property - Source Types to Processed.
In this case desktop will not apply a stretch to the service by default.
If you want uses to have the option to select a range of different enhancement and have something else as a default then use Raster Function Templates (available in 10.1) that enable a range of functions to be associated with a published image service. You can define a default function to be applied. AS above set the Source Type as Processed to stop Desktop from applying an additional stretch.
Alternatively in ArcMap Options,Raster, Raster Layer there are options to control custom rendering defaults.