I have for the last several years used both top and bottom entrances in both the winter and summer at the same time, with a 1" pink sheet of foam insulation over the hives and nucs alike during the winter, as I winter my hives near Buffalo, NY.    I do however always put a two pieces of 1/2x1/2 scrap wood across the top bars(giving the cluster a place to sit) then place a sheet of paper with about 5-10 lbs of sugar on the paper with a pollen patty.  The paper and sugar absorb any moisture, plus it is any easy access food supply for the cluster if need be.  If there is any sugar left in the spring i just use it to make syrup for early feeding. (all my hives winter over in double deeps with the top hive body being full of honey).  Granted I am only wintering over 50-75 hives.  Just a side note, I have lost 3 nucs and two hives so far this winter, with one nuc being a deadout and the others missing.

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