The Use Year serves two primary functions:
1. It determines when you receive your annual allotment of points, and
2. It determines your annual point banking deadlines.
With regard to banking, you can bank 100% of your points in the first 6 months of your Use Year, 50% in the next three months and 25% in the next month. In the final two months of the UY you cannot bank points.
The best UY is the month immediately PRECEEDING your normal travel patterns, assuming that you actually travel at roughly the same time each year. If you travel a different times of the year, the UY doesn't really matter.
Here's an example of how it CAN matter:
Let's say you have 150 points and use all of them to book a trip in May.
Now, let's assume that you discover in March that you need to cancel your trip. 100% of the points will be returned to your account since you are not within 31 days of check-in.
However, let's look at the impact with two different Use Years:
JUNE - June UY points need to be used by the following 5/31. If you need to bank 100% of the points, it must be done by 11/30 (first 6 mos of the year). You had intended on using your points for a May trip. By the time you discover in March that you need to cancel, only 25% of the points can be banked into the next year. If you are unable to re-schedule your trip before 5/31, you will lose a large portion of the points.
MARCH - With a March Use Year, points must be used by the following 2/28. You have until 8/31 to bank the points if you won't use them. Thus, since you actually cancelled in March, you have a lot of time to decide whether you will use the points or not.
Again, I just want to stress that if you travel at different times of the year, then UY really doesn't matter. But, if you ALWAYS intend on using your points for Spring Break, a June UY would be a very poor decision. You would always be running up against the end of your banking (and usage) deadlines for any unexpected cancellations.