I think most guys would agree with your logic that you need to run all 6 at one line length prior to setting a PB at the next. How many stabs you take at each line length in the process is really up to you and whatever you're comfortable with.
You'll find various opinions on this thread I'm guessing, but the beauty of a practice PB is it maybe shows your potential more than it shows the level you're at now. Set a practice PB (multiple attempts allowed), then try to ski consistently enough in the future to set it in a tournament. Or at the very least run down the line in practice to do it without missing any balls. Either way, have fun with it.
Bottom line…girls at the bar aren't really impressed with slalom scores and PBs, practice or tournament. It is what it is...