slate baking stones

I've just acquired two pieces of slate to use in my ovens for baking bread and pizza.  I'll have them cut to size 14" X 22" today (my ovens are 24" wide).  That will give me an inch + all around.  My question...........Do any of you leave your stones in your oven all the time, or does the repeated expanding and contracting with the heating and cooling lead to their break down?  Also, do the stones impede or alter in a negative way anything else I would cook in the ovens? 

I have a whole slew of grands, and I'm really excited about being able to make two pizzas at a time in each oven.  On the other hand, I'm having visions of my poor old granny back hoisting those things in and out.  I do have a close to the oven storage place for them if needed.  They'll stand on end between a deep freeze and the wall.  I guess I should ask if that's OK, too, or should they be stored flat?

Syb