My views:
If the ATIS says it's wet, then by all means, I'll count it as wet. It's often impossible to judge from the apron, the surfaces differ too much. A contaminated apron may well be a wet runway (admittedly though, the runways usually allow for more water to run off than the aprons and will therefore often be in better condition than the apron). My point here is that it is difficult to judge from the apron or gate, weather or not the runway is in fact wet. So if the ATIS says something, I'll use that information. It should hold up in court.
As for using wet or dry figures, your SOP should provide guidance. My SOP tells me to use dry when dry, wet and dry when wet (and use the most conservative value) and when braking action reported, use both BA, wet and dry, and when contaminated, use contaminated, BA, wet and dry. All of it is paperwork and manual calculations so it's a bit of a chore on those snowy days...
Because of the various special cases that arise with the different modes of calculation, by allways using the most limiting value, you "should" be on the safe side performance wise. I haven't, though, given this any deeper thought if this is truly the case, but it sounds reasonable to me.
merry x-mas!
/LnS