I have always done weekly weigh-ins, which has worked well for Run With Jess's Lose a (Half) Marathon challenge. However, here lately, I've been weighing in more than that, mainly because I've been making a lot of bad food choices and my runs have been sporadic, at best.
I haven't decided if that is good or bad for me. Once a week is plenty, but seeing the day-to-day fluctuations reminds me that one bad weigh-in doesn't mean my weight loss goals are ruined. I think too often can be a problem, though, because our weight does roller coaster up and down so much. It's easy to get frustrated with that.
I think it all depends on your mindset. If you can handle the up and down, then whenever is fine. But if you're like me, once a week is more than enough.