I like RevolvingGarbage's picture of the Iver Johnson. Even more I like the point he's trying to make. If the gun's trigger takes 12-15 lbs of pressure to make it fire, putting a non-animate object in the trigger isn't going to make it go off. Even if the whole weight of the gun were resting on the knife, the gun does not weigh 12-15 pounds (or even close to it).
Personally, I don't care for taking pics with things in the trigger guards because I think it detracts from the point of the picture, the firearm showcased. I don't mind if it's displayed as though it's an everyday object with other things laying round it, but usually the thing IN the trigger guard has to be something completely unrelated or non-associated with the gun to be able to prop it up. (I hope I said that clearly).
I personally try to prop up my guns with something behind them and angle it so the object is invisible in the pic. That's just me though.