With regard to what Qmaster said about the grid vertex snapping.
I believe the editor should have a preference for this.
I think it's similar to how I could do this in Quest.
In Quest, the config file has this option:
# ---------------
# snap_to_int <x>
# ---------------
#
# If set to 1, Quest will snap all vertices to integer values upon save. This
# should prevent vertex drifting. This is only used with the old map format.
snap_to_int 1
---
There are couple more settings in the config file that you might find interesting. See quest.cfg:
http://pastebin.com/7CCDm8R6