We always have done so for things we can’t safely post on Tumblr.
Tumblr has more restriction on age and species stuff than Derpi does. So if Diamond Tiara is possibly under 18 in a pic or someone gets offended by Applejack rubbing her butt on a human or something we’re covered and don’t have to worry about being reported for something.
A few years ago some nuts ran around reporting everything to the point where blogs would get deleted without notice and so on. We’re just playing it safe because of things like that.