Hi Vince-
 
Here in central California, I overwinter four and five frame nucs every year
(not mininucs-I think there is a terminology difference here).  I typically
end up with fifty or a hundred very weak four and five frame nucs (full depth
frames) every September in the course of late carrot and onion seed rentals.
 I take them to a warm location near the coast, stick a syrup bottle through
the roof, and requeen them after a week or two.
 
By October or so, I transfer the strongest ones into 8-frame full depth boxes
to overwinter in.  During late December or early January, I super those that
need room.  If they have enough frames of bees by February, I rent them in
the almonds.  If not, I let them sit some more or rob them of enough brood to
knock them back into four or five frame nuc boxes to rent again in the early
broccoli and cauliflower crops.
 
If they were too weak to transfer into full size boxes, I leave them in the
nuc boxes all winter.  This last winter I lost three or four out of about 30
that I didn't get around to transferring.  I lost more to swarming (couldn't
get to them through the mud) than I did to winter losses, although the skunks
didn't help any.
 
It's too cold where I live to keep the nucs alive, but by taking them to the
coast I can keep them percolating all winter.  Some that I transferred out of
five frame nuc boxes in September made 12 frames of bees for the almonds in
February, and were split in early March.
 
Obviously, the key here is the mild climate.
 
Kevin