FWIW, my cross pin is not splined either and if there is no pressure on it, it is a slip fit. I am an engineer by trade, and well understand the mechanical assembly of the rifle. I was uncomfortable with how much the cross pin tended to shift, and while it may just be resting there and might not cause me any headache, the anal retentiveness in me was why I went ahead and made the endcap.
Also, I recently took the action out of the stock to clean the trigger group (the dirt and dust from many camping trips had it a bit grimey), and I see no corrosion of any sort between the two metals. Not to say it won't happen. Maybe the fact that the action is blued provides enough of a barrier between the two metals that they don't corrode; I don't know. I'll certainly keep an eye on it.