The PA28 manual (and presumably the PA32 manual too) recommends retracting flaps as soon as the aircraft is on the ground to reduce lift, increase weight on the wheels and assure braking action is effective.
We teach this from the get-go with students.
However, the terms 'braking action' and 'wet grass' do seem mutually exclusive.
Careful reading of this report does suggest that in heavy rain, a paper type air filter can become saturated with water to the extent that air supply is cut off. In this case in an injected engine, 'alternate air' should restore flow, like selecting carb heat to 'on' bypasses the filter with a carburetted engine.
TOO