Always kept all my keys together. Never even thought that it could damage the ignition housing. Never had any problems with any of my cars though in that regard.
Happened to my Mum's RAV4, then again the way she had her house key and couple other keys on it was no surprise.
I've got all my keys on one fob; don't know if the keys are different with age, but I've got a 60 plate BMW so got the card-like key, not your typical key, and it doesn't move at all in the slot so I'm confident it'll be fine forever*.
*Car's lifetime.