So can we say - sing whoever's songs you like - but be sure to attribute the song to them preferably BEFORE singing the song on stage, but,if you want to record,do your level best to get permission from the Songwriter / or the Song copyright owner.
Greko also speaks for me when - to quote - "would be proud to have their songs sung/recorded by others " after all it DOES constitute the most sincere compliment which a Writer could receive.
Good Luck.