I’ve not seen any information on which anniversary date has the most points associated with it. My guess would be the month that a given resort opened (you can find that out at http://dvcnews.com/), but that is just a guess. If no one here has an answer, you might want to try at http://www.mouseowners.com/, a board that is dedicated to DVC.
At any rate, I don’t think that it really matters if you match the anniversary date when you buy additional points. With banking an borrowing you have quite a bit of flexibility to deal with any timing issues that may arise when trying to use points from both contracts.
@len makes a good point about looking at the total cost when comparing resorts (initial cost plus annual dues). If you are buying resale, also check to see how “loaded” the contract is, i.e. how many unused points from the current year are included.