I agree that normally preheating is not required (or desirable) for welding austenitic stainless steels. Preheat may however be required for other stainless steels in the martensitic series. It is therefore necessary to know what grade of stainless is being welded before a definitive reply can be given.
Now to my point - I've dealt with some (well, one) companies who insist that 100C preheat is used on stainless steel as they claim that it helps the weld run better... I am sure that this is probably due to an influence on the cooling rate of the weld and hence spread of the weld pool. I've not actually seen the difference myself. It's been difficult to try and convinvce them otherwise.
The only advantage to light heat preheat on SS is to assure that it is dry and that the solvent that you used to clean it has evaporated.
Metal temp can be an issue with austenitic stainless if the material is cold. If ambient temp is below freezing then some preheat will help the welding.
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Rust never sleeps
Neither should your protection
Other than the aforementioned elimination of surface moisture the only preheat we allow during the welding of SS is applied to the welder. I've found this to be great inducement to speeding up the welding process.
As stated before our site is primarily Austenitic SS operating within a process where IGC is the predominate corrosion mechanism. Even with the advent of the L grades we initially tried to limit the heat input to a weld by requiring stringer bead welding. As chemistry of both the materials and weld consumables improved we started cutting a little slack by allowing weaving in weld process. We have found that now the weld bead configuration has very little effect on the integrity of the weld and the HAZ. The last big concession we made was to allow a closed root with "walking the cup" torch control.
