The only time, IMO, to get very particular about a specific UY is if you usually travel one time of the year, every year. If so, IMO, it's most beneficial to pick a UY that is immediately before when you will usually be traveling. The reason is because it will give you the ultimate flexibility in case you might need to cancel a trip. If you are going to travel throughout the year, then it doesn't matter. (Though some months are more common and might be easier to pickup some resale contracts for the inevitable add-on).
You can borrow points from the next UY regardless of what month your UY is. If you went with a June UY, you could borrow points from June 2020 UY for an August 2019 trip.
Another consideration is that when you buy direct, you'll get the current UY points. So if you got a June UY, you'd have UY-18 points and then June 1 would get UY-19 points. And they will let you bank the points from a direct purchase, even being way outside the banking deadline.