My concern would be the quantity of bees still remaining. There must still be a big enough cluster of bees to prevent the population from dwindling to below a warming critical mass. If your temps provide, I would try to evaluate the cluster size. I would also check to see where abouts in the hive the cluster is located. In reference to their location, where and what quantity of stores remain, are these stores enough to sustain them until your spring. If the cluster is small, try to make provisions to assure the bees have to work as little as possible to stay warm (make sure SBB are closed, perhaps tape any questionable seams, or wrap with tar paper to get some solar gain as the days grow longer). Keep in mind, as the cluster is kept warmer, so will it also consume more stores (and that doesn't even consider the soon to come brood build up and consumption). This is way it is important to evaluate the stores. Depending on your season, you may need to supportively feed (liquid, dry sugar, or candy board) based upon your temps and personal beliefs.
Currently it is mid-winter for me, ok it's mid winter for everyone but its the middle of the worst cold (some people get a late start on the cold, others just experience rain, etc etc). For my area, I can't chance a wet frozen cluster so feed must be either dry sugar or a candy board. I like candy boards because they provide just enough room to also pollen feed under them. Pollen could also aid in your situation if the population is tapering to a critically low level (it stimulates brood rearing and would increase the population (provided enough bodies are available to keep the brood warm until emergence).
Some folks will also move dwindling hives into nucs to minimize cold exposure or set the hive on top of another stronger hive for some added warmth. I can not say that I have found either to be the saving grace needed to save hives. My ideals are the bees have laid out their combs for the winter in a certain pattern for a reason. This reason is beyond my own reasoning and any changes I make may require the bees to re-change (consuming extra energy that the compromised hive direly needs). The best I feel I can do is support their decision with feeds to keep them energized and in good health (regardless of population).
Hope this helps, and that it doesn't instill any poor or incorrect beekeeping morals I might have.