In my view there are ways and ways of 'messing with the rythm'

If you are singing/playing and you want to stretch and bend for 'artistic effect', well it's up to you. (Others may like or loathe your interpretation)

If there are dancers and your playing, you'd better keep the beat going or you'll trip them up (unless it's Morris 'slows' when tempo is dictated by how high the dancer can leap, I think)

If someone has started a tune in a session and you don't keep to their rythm, then iyou are being rude - and if they are playing it in an 'expressive' way and you can't join in, then it had better be a solo.

My personnal moan is that many tunes (e.g. waltzes) have a note at the end of the line that shoul last 2 bars, and some session players cut it short.