There's no painting rule that says you must varnish an acrylic painting. It's a matter of personal choice. Personally, I believe that any painting that is not going to be framed under glass -- which most acrylic paintings aren't -- should be varnished to protect the surface of the painting from dirt and dust in the environment. When a painting is cleaned, it's then the layer of varnish that's being 'washed', rather than the paint.
Use a removable varnish (the bottle will tell you if it is) so that if it yellows it can be removed (though this is less of an issue with 'modern' varnishes based on acrylic resins than with 'traditional' varnishes).
See Why Varnish a Painting? for further info on the subject of varnishing.