I used to watch blue-table painting's battle reports a lot, and in almost all of their fantasy games they used a house rule they called "protective terrain", which worked as follows: 'if any model in a unit is completely within a piece of terrain, the unit as a whole gains a 6+ ward save. If they already have a ward save (including parry and regeneration) they gain +1 to that save, to a minimum of 2+' They said it represented the models dodging behind a tree of busted wall or whatever to avoid a hit. I just wanted to know what you guys think of the rule, because i wanted to introduce it to my group.
I think it should probably function like a parry save. Units with out hw + shield get a parry save. Units with a parry save save on 5+. I don't see it logically stacking with Regeneration. Or most ward saves which are mystical in nature. Savage Orcs for example don't fight that defensively and rely on dumb luck. Phoenix Guard already move crazy fast. Slann wouldn't be likely able to use trees to dodge.