Going to have to agree with the others--I've not really heard of marital status being used decisional factor. The one situation I can think up where it might come into play is if a POI is debating between two applicants, is worried that you as a top choice may go somewhere else, but knows/finds out that your spouse is also applying to the program and thus uses that as evidence that you'll be more likely to attend.
But again, I've not yet heard of that happening. Not saying that it doesn't, just that in my experience it isn't at all commonplace. I'd say just get married whenever is best for the two of you. Doing so during the first couple years of grad school could be tough, though, so if you'd prefer to be married sooner rather than later, then doing so before beginning your program might be best. Otherwise, it's possible you'll need to end up waiting until you're a bit further along in grad school, once classes calm down, you become more accustomed to the general atmosphere, and your schedule becomes a bit more flexible.
