Well, for my sins, I used to do the last clip for the girls ponies during the week before Feb half term. Just at the time when they needed a quick smarten up prior to going out and about.
It always worked for us and we never had a problem with their coats.
I changed Farra from a chaser to full and went over Chancer's full clip a week ago - I normally do this last week of Jan/first week of Feb to have a nice show coat by April.
I have clipped as late as end of Feb in the past on my TBs that kept their coats to late and still had a nice show coat by May.
If you don't show, then it doesn't matter at all when you clip. I clipped Cairo all year round and have done one of my show gypsy cobs in late July and had a lovely fine coat again for my big show in early September.