Reno, Prepunched k.o.'s probably cost more labor time over the years than any other annoying thing. Most electricians would agree with you that they offer little benefit especially when using strut to support conduits.
The handiest method to blank off k.o.'s I have found is to use a 4" square blank or other piece of code gage metal bolted over the knockout. Good idea on the reducing washer too!
