I've changed the blog settings to allow anonymous commenting and log only the IP of anonymous commenters. Not that I put much stock in IP logging anyway, but maybe it'll be a data point with troublemakers who are careless or technologically unaware. I might turn off anonymous commenting if it turns out not to be worth it--I turned anon off on my main Tumblr for a reason--but I won't be logging the IP of logged-in commenters again. That was a case of apathetically going with the default, not a really conscious decision on my part.