Not so much positive earth, but many devices have 'floating' wall-wart supplies, and some (presumably audio and serial devices will be the main offenders, as those signals go negative with respect to ground) assume that signal ground will be somewhere in the middle. They're going to object if you go and ground[1] either side of the power supply, as they'll no longer be able to swing in both directions.
Not the sort of thing that's easy to predict, so if you're playing the LV distribution game, at some point something's going to want a DC-DC converter on the supply to make up for the lack of signal isolation.
Kim.