depends on when you travel. generally you'd like to be able to travel primarily in the first few months of your UY. (and not have to travel at all in the last 3 months of your UY). but it's only an issue if you have to cancel.
The general theory of UY is that you want one in which the bulk of your travel will fall in the first 8 months. The reason for this is because of the requirement that all point banking must be done within the first 8 months of your UY. If you have a trip booked in that window and you have to cancel it, you will be able to bank the points in question (except if they go into holding, but that's a separate issue). However, if you have to cancel a trip that falls after the banking deadline after the banking deadline, you're stuck with those points in that UY.
Thus, if you like to travel during the fall and winter, something like a September UY is perfect, because things like Adventure season, Food and Wine, MNSSHP, Christmas decorations, New Years, and Winter/Spring break all fall within that 8-month sweet spot. If you're more a Winter/Spring break and summer vacation traveler, then something like a February UY will be better for you.