Rich,
I understand that the runner does not need to tag up until the ball is caught, but based on Senor Azul's post,
"If a runner is forced to return to a base after a catch, he/she must retouch his/her original base even though...."
In our case, the runner started to head back to first but the ball was overthrown out of play with the runner never having touched first.
I didn't expect the tag up to happen before the overthrow. The overthrow happened while the runner was headed back to first but never got there. He got about 2/3's of the way back to first and then the first base coach sent him to second after the overthrow. The head coach argued that it was an overthrow and should be two bases. We argued he never touched or tagged up at first to warrant the awarded bases.