"Open as needed"
It is common practice at our association apiary to leave feed holes open all year round, with no (dramatic/noticable) effect for the last 50 years.
This is what I during my first season (ie spring/summer/winter), as as a previous poster has said, with no problems - the hive came through winter just fine.
So, anecdotal evidence shows open cover holes does not kill bees.
What I did notice was the roof was damp, and after joining here and reading perfectly reasoned and scientific advice I decided, primarily to prevent the damp, and to reduce the amount of stores the bees used over winter, to INSULATE above the CB - which effectively closes it. Not to save bees lives, not because it was a huge faux pas, simply to increase the odds for the hives to make it through winter.
As it happens, one hive starved itself, the other superceded at some point and I had a DLQ.
Goalpost free facts; with open feed holes over winter, 100% survival rate. With closed feed holes, 100% loss rate.
However, of course, these things happen, and bees will survive, or die, despite what we do to them (within reason), and open feed holes is within reason.
What do I do now? Open most of the time during the summer as I have them cleaning frames/comb, and INSULATE over winter to help prevent damp and reduce the amount of stores used.
In my view, covering feed holes has negligible effect, INSULATING does however (summer or winter).
Until such time as I hear a reasoned argument to change these methods, after all I am just a humble beginner beekeeper who wants to learn and is more than happy to take up new methods.