>It is good idea to test just opening hives in winter vs not. It might be tricky. 

With our setup, with the summer lids used over pillows, we always checked hives 
for survival and condition in January, and I think that many commercial beekeepers 
do the same, especially those with one-piece wraps that are easy-off and easy-on.

I usually did not pull frames, but often looked into many or all hives in a yard in
February to determine the likely spring numbers.

When the cluster was on top and the frames loose enough to pull without fussing,
I often puled a frame or two in the occasional hive to check for brood, and where the 
cluster was obviously away from food, made adjustments any day of the year that 
I was comfortable without a hat and gloves and when conditions were calm.

I was never able to detect any harm.  Rob Currie, however did report that in his 
work where he was having to move frames to place sensors (if I recall) into 
wintering bees that something he was doing lowered the apparent survival.

Opening a hive can be like open-heart surgery and not something to do casually
and without consideration of the possible consequences, both good and bad, but
that is what most beekeeper like to do -- open hives and look into them.

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