For most folks, use year isn't that big a deal. But there are at least a few instances where it can matter.
1) If you have to cancel a reservation that is in the last months of your use year, it can be more difficult to use those points than if your reservation were in the beginning of your use year. The worse case scenario would be a reservation in the last month of your use year that has to be cancelled more than one month out, but less than four months out. While such situations are rare, the do happen and are annoying to the few affected. So if you know you always vacation in August, all other things being equal, you would choose an April use year over September use year. But don't obsess over it.
2) There is also an issue at purchase. It can be better to buy before your use year. If you buy an April use year contract in August 2008, you might just get 2008 points. But if you buy a September contract, you could get September 2007 points and then another set of points in September 2008. It won't necessarily work that way - you have to look at the specifics of your deal - but it's something to pay attention to.
Note that while points show up in your account on the first day of your use year, you can make a reservation that uses those points before they show up. Only the 7/11 month windows apply.