In American Appalachian or early country music and even in this of the 20's, open fifths were a characteristic of what the bluegrass's call "the high lonesome sound".

To answer the original question, I would opt in favor of 3rds, 6ths or 10ths in harmonizing in a jam session. The use of 5ths would be a judicious one.

I like the way Ewan McColl and (I can't think of his name now) sing "Go Down You Blood Red Roses" in 5ths. It gives is a stark and rugged effect. The original version of "Sometimes I Feel Like A Motherless Child" was collected as a so-called "white spiritual" with open 5ths reminiscent of shape note singing which use a lot of open 5ths.

The old joke is: whenever you find four Episcopalians together, you find a 5th.