I can see a case for both and I guess it comes down to whether you see a Playlist as something you are going to follow incrementally, or whether it is just a way of curating a list of songs that you might choose to bounce around in.
If you are still largely going to work through your Playlist incrementally, to me it makes sense to still have the next song selected/highlighted so that you know what song will load if you hit Play.
If you are more likely to bounce around a Playlist, maybe as a safety feature to prevent accidently loading of a song (which might have on-load MIDI events), then NOT selecting any song is both more accurate to the way you work (because the next song in the Playlist is not likely to be the next song you play), and safer. When I say safer, if the next song in a Playlist is selected and you accidently hit Play instead of scrolling to another song, then it prevents this. Maybe that is a bit of an Edge case however.