I've been working on my own wedding contract for future use, and I have down 50% on booking, 50% before proofs are delivered (with it recommended being paid the day of the wedding). My justification is based on two things.
First, I think splitting it up too much (more than 3 different payments for the basic package) is annoying and requires more of a calendar than I think the couple wants. So I'd say stick with nothing more complicated than 50/50 or 33/33/33.
Second, I went with the standard for the first payment, basically a "hold the date" payment. But for the second payment, I thought to go with the "before the proofs" more as a good faith gesture to the couple, saying, "I trust you to pay me for the work I'm going to do." BUT, they don't get any of the photos until I get my money, hence the second 50% due prior to receipt of the proofs. After all, by that time, I've finished pretty much most of my work, and so should have all my money.
Edit: If they opt to order anything in addition to whatever package prints, then payment is due upon ordering, as with pretty much any other kind of merchant (think ordering from a catalog, if people actually do that anymore).