The answer is a resounding yes that UY matters. It's insurance. How much it matters depends on your travel habits. It can easily be worth a years worth or points or more. For those with defined travel patterns that aren't spread throughout the year, 100% of them should pay attention to UY. For those that travel throughout the year routinely and fairly evenly, it's less important but realize it's because they've given up the protection and that DVC is inherently more risky and possibly costly for them. For those that don't have a good idea of their travel patterns, they shouldn't be buying anyway in all likelihood.
If your usual travel is about 50/50 between those times, for a single UY Sept will be best because it is the UY that gives you the best protection for both times. August and December will be decent as well but definitely less protection. The next UY before that are June/Oct so they remove a full 2 months protection from BOTH times. For that scenario and assuming it's not planned to change such as having younger kids and you'll need school calendar trips later one, I'd only consider those 3 UY in your situation even if you have to wait longer and/or pay a little more and truthfully, I'd personally wait on Sept even if it means you miss the first trip you're planning. Lets say you plan a 7 day trip that has 3 nights in Jan and 4 in Feb and your UY is Feb. Then you have to change or cancel in Dec. Your only options with the points from the 3 Jan nights would be to rent them, use them before Feb or lose them. If you had more time with a different UY, you'd potentially have more personal options as well as potentially being able to bank them. Again it's insurance on this situation. Now obviously some have far more or less stable situations than others making the risks less or more but everyone has risks. Not all risks can be removed by choosing UY wisely but much of it can. There are other strategies on the booking or renting side that can raise or lower risk as well.
