After the Terminals were removed from the timeline, the Guardians of Forever took over their masters' work ensuring that the timeline remains fairly stable. That being said, they are more lenient then their predecessors. They don't really care if you mess with the timeline to save a loved one, or help out a family member in the past, so long as nothing you do has a serious effect on the big picture.
They draw the line at anything that has massive repercussions, and if you DO manage to do something that serious, they will usually find a way to undo it. For example: killing Hitler. This has actually been done several times (Actually, the first time Hitler was killed, it was not actually Hitler, rather someone by the name of Helmut Schenk), resulting in the Guardians keeping a cloning facility outside of Hamburg (They don't LIKE cloning Hitler, but they do it to keep reality somewhat stable), they actually give tours to visiting Geniuses. If you deliberately infringe on the timeline in a way that effects anything more than, say a village, you can expect a nice visit from several Guardians who will proceed to, has the sourcebook says "kick your ass" and keep you from mucking up reality too badly. Please note that the Guardians are very powerful and you will likely lose in a straight fight with them.
Lately though, the Guardians have been growing more corrupt and listless, being assigned to time period for millenias can do that to you.