I'm new to the fixie/ss world but I've been in the roadbike world for awhile now... there are kinda two theories:

1) being that some tires with big nobby tread is best because the nobs grips better
2) thin road tires cut through the slush and in the end grip better becuase they don't get packed off like mountain style tires

I personally use the same road tires year round.