"...I won't know if they're really what the song needs until afterward." That idea perplexes me.
As an aside; Have you ever watched the video about Tom Petty and the Damn the Torpedos album?:
""Jim Keltner was in the hallway with the shaker," he (
sic Tom Petty) recalls. "He was standin' outside the door playin' this, and I came out and Jim said: 'This is what that track needs.' So Jim Keltner came in... and put this shaker on, which, believe it or not, if you put the drums on [
adjusts the sound] it's okay, but it's not really got the mojo, you know. So, Jim Keltner, we owe him a lot.""
Anyways... I'm just throwing out an idea... not looking to force a disagreement.
All the best,
mike