The second(2 legs tied together)is considered best practice.
They are functionally the same.
My understanding is the major difference is if the pot fails, usually the wiper (the moving part...), the first layout(leg floating), you get an open circuit. Among other things, harder to troubleshoot.
Also, it's not good to have a portion of the circuit floating and ungrounded. If one leg is left dangling in the wind, you have a voltage divider with signal on one side and radio stations on the other.
