No PL on 19 or 20 simplex. If using repeater channels, PL 141.3 is generally used for open repeater systems, however not guaranteed.
If you program a PL on simplex, you won't hear other traffic on that channel unless they are using the same PL. If you are scanning for traffic, it's best not to program PL because you will hear all traffic, even if they are running any PLs.
Hope this helps.