So many good points here, especially the one at the end, "make sure the builder understands your intent" and I would add that it is more incumbent that YOU understand your intent and be prepared to understand when the builder won't, can't or doesn't want to do something, they may have a workaround for it that you won't like.

You very much need to let them do their thing, this is how the magic happens, if you get them to do things they don't want to do, it may skewer the big grin, I love this bike, in the end.

I would encourage anyone to go down this road but only if you realize it needs to be a mostly %100 positive process to be a %110 positive outcome.