The simple:
It is probably in your best interest to travel in the beginning of your UY (within the first 4 months or so), and NOT in your best interest to travel in the last 4 months or so of your UY. This is based on the idea that you have somewhat typical travel patterns.
The longer explanation:
You can bank your points at any time in the first 8 months of your UY. You CAN NOT bank points in the last 4 months of your UY.
Let us, for example, say you plan on going almost every Thanksgiving. Not knowing any better you buy a December UY.
September 1st hits, and for reason X, you are going to have to cancel your Thanksgiving trip. It is passed your banking dead line for a December UY, so you can not bank your points. They expire November 30th. You now have 3 months to use them or lose them.
Same thing, but cancel November 1st - Your points are now holding points, and expire in 30 days.
Now let's say you were "in the know" about UY, so, since you plan on going almost every Thanksgiving, you bought an October UY.
Back to Scenario 1, cancel on Sept 1st. Points go back into you account, and you now have 9 months until your banking dead line to use them or bank them.
Scenario 2, cancelling on Nov. 1st. Your points still go into holding just like before, so they can not be banked, and holding points can only be used 60 days out. You have until September 30th to find a time to use them (the last day of your UY)
Hopefully this clarifies it some, and illustrates the idea that it is extra insurance for your points, to vacation earlier in your UY.
When points expire they are Broken (go into Breakage). If you travel late in your UY, the chance of breaking points due to a cancellation rises. If you cancel a reservation early in your UY, your have more wiggle room with what you can still do with those points.
The 'IF' being if you tend to travel during a certain time frame. If you are going to make reservations all over the place, then it really does not matter