I have never nailed on a butt plate, but know that you should never use a square peg in a square hole for this sort of thing. Your holes should be round. This is due to how the wood is compressed around a square peg. This rule holds for wood, but I can't be certain whether or not it holds for metal into wood, but I see no good reason it shouldn't.
I have observed construction of old barns in my area 30 years ago or so, and did notice that they often used More like hex wooden pegs into round holes. The barn that I grew up with/in was build in 1839, totally of wood, except for the stone and mortar foundation. This barn had this type of pegs holding it together, and my father, being worried that the pegs might be loose, tried to pound a few in. They would not budge. This barn had seen 150 years, in an area which saw high winds at times, temperature extremes, and lots of weather. It was solid as when it was first built, and probably stronger since it was originally made of native lumber, which gets very much harder in that area over time.
Matt