It mostly has to do with banking deadlines.
Let's say you have a trip planned for Dec 25-31 using your current year's points. On November 15th something happens forcing you to cancel the trip. You're still more than 31 days prior to check-in, so you have no problem in that regard. However, you are only about 75 days from the end of your Use Year. At that point, you can only bank 25% of the points you own. So, you would be in a position where you would have to do something with those points before 1/31 or risk losing almost all of them. 25% of your total ownership could still be banked (assuming you haven't already banked any), but the rest would be in jeopardy.
With your vacationing at so many different times of the year, I don't know that any one UY would be better than another. The Feb UY works great for your May trips--if you have to cancel a May trip you will probably have 8+ months to either use or bank the points, and that still falls well within the window at which you can bank 100%.
Don't sweat the Use Year. Just be a little more careful when planning those December trips.