The best time to cut timber is when you can get a logger to cut it, lol.
I've had a contract signed since Spring and still not got it cut.
Seems the loggers in my area are booked up way into the future. I talked to one local logger and he said that he would do the job but it would be almost two years before he could get to it.
I don't think it matters much if the "sap is up" as long as the timber is going to be cut milled and dried ASAP. Timber prices might be higher in the Winter, i'm not sure and don't really care as I am not as worried about the cash as I am about improving my deer habitat.
There are a few "loggers" on here that know a lot more than I do about the subject i'm sure they can and will answer your question when they see it.