"Is there any kind of etiquette at all? That if a singer sings a song regularly it is polite to leave it alone? "

I was at an Accordion Festival. I was waiting my turn in the 'walk up and play' section, when a pair did the tune I was planning. But their version was so different, done on different instruments (banjo & hammered dulcimer vs my piano accordion), and differently paced, that I said 'what the hell' and did it anyway.

They came up to me afterwards, very impressed with my version. We both agreed that our renditions were so different that neither of us had any real problems 'stepping on each others toes' musically.

The tune "Ashokan Farewell" - originally deemed to be a 'violin solo' :-)

BTW, I now prefer to be earlier in the 'walk up and play' section...