Hi, have you seen this: http://world.episerver.com/forum/developer-forum/-EPiServer-75-CMS/Thread-Container/2014/7/Setting-Update-modified-date-to-checked-by-default-for-all-Saves/
I usually recommend using the save date instead in these cases. It's updated everytime the page is saved (duh :) ).
I'm in a project using EPiServer 7.5 and I would like to update the Changed date each time a new version of a page is published. I would like this to be the default behaviour without the users having to manually check the SetChangedOnPublish checkbox. I've managed to fix this in EPiServer 6 once but in 7.5 it seems like a different story. Any ideas?