....Andy, it depends on your goal. If the holster is designed to handle better being rigid, then I advise against conditioners. A holster starts it's life as too tight, breaks in, gets too loose, then finally wears until it's flabby and way too loose to be effective. Anything that speeds break-in too much, will shorten the life of the holster.
If it's simply too stiff and you want it softer, then use a conditioner made especially for a gun holster. They are void of water in the conditioner...rust waiting to happen otherwise. I have always put my gun in a thin baggie at night and just crammed it into the new holster. During the initial period I carry the gun in the holster normal style during the day. It won't take long for the leather to stretch and be form-fitting. I have a new Kimber in a baggie in a new and very tight Mitch Rosen holster...sitting my in my gun safe as I type. It is already loosening up and feels much better. The holster is custom molded for the Kimber Pro model, so it's tight initially by design. JMHO.