A lot depends on what part of the country you live in. In Texas I would never store any kind of gun in a holster, but in Colorado I don't have a problem with storing in a holster. It all depends on the humidity level in your area. I would add to make sure the leather is vegetable tanned.
One thing you might try is to polish your firearm with auto polish such as 2000 Washes or Nu-Coat. Polish it inside and out, including the barrel and stock. This is what I came up with when I lived in Texas and got tired of sloshing oil on my firearms. This is a long term storage method. You can remove it with Hoppe's #9 or a similar solvent.