I never know what to re-publish anymore.
Most recently: I rebuilt standard previews then restarted Lightroom, now there are 9444 photos "modified" to republish (there were zero shortly before building standard previews).
There are other times I go to publish new photos or honestly modified ones to find anywhere from 1 to thousands of photos in the modified to re-publish section, which hadn't been so modified.
Also, marking them up-to-date is still problematic (takes multiple tries) - like in the beta. reminder: that was supposedly fixed in Lr5.2RC - it's not.
Rob