Probably good idea to prevent flaking of the paint. I suppose acrylic will stay flexible on the polyester cloth when rolling it up over and over? I wouldn't want to fold it after painting, because it's always got creases that way.
Discussion
the fabric medium is what makes the acrylic paint more flexible so it doesn't flake or crack.
your best bet is to do some testing samples. I haven't done a banner like this myself so that's what I'd do first.