I doubt the idea or concern about 'slides' being in or out during storage makes any difference one way or the other, though folks will certainly give you their 'reasons', for sure...
if you are 'living' in your coach, parked for months on end, the slides are always out, right? why would it make any difference if the coach is parked or stored for months on end, with no one living in it? None.
Some folks have rooms to leave them out during storage, and it gives you more room to move around should you need to visit the coach periodically. Some folks don't have room, or the coach is exposed to the weather more, and they feel more comfortable with the slides in. To each his/her own, it really matters not.