In or out, I think it's more a personal preference. If left out, there's more "outside surface" to get dirty and all of the exterior slide seals are exposed to UV degradation. Be sure to keep them coated with seal conditioner/UV protectant to help preserve their longevity. On the other hand, if store in the travel position, be sure to cover the tires (if under the slides) to protect them from UV degradation.
Keystone, in the owner's manual, recommends that the slides be "functioned" on a regular basis. If left in the open position, they recommend 1-2 times weekly. The owner's manual also recommends that trailers be stored with the slides retracted.
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS