I stop turning at the end of day 13 because the Cobb development charts says that day 14 ish is when chicks make their turn toward the air cell end. Eggs do not need to be turned after two weeks because they can move themselves and there is no danger of "sticking" to the side causing development problems. My last hatch (20 chicks) I had zero upside malepositioned chicks. I didn't have a problem over all with malepositioned, but it wasn't common to have at least one per hatch. I feel that the less turning at time of the chick turning can only help them complete that process.
I believe that people generally choose day 18 because this is "lockdown" for a majority and the greater emphasis on hands off and not opening the bator, limits the ability to turn after lockdown. So, everything is done at that time. I myself don't subscribe to the hands off theory, but I do raise my humidity at day 18.