I have some bio that is washed and dried but has been sitting in my garage with the top off. Will I have to dry it again before using it? How critical is it to keep finished bio in a sealed container?
Yes if water tests show it's wet.
How humid is the air where you are storing it?
In my climate, which I have to admit is normally very wet, biodiesel will absorb approximately 50 ppm of water per week in an open topped container. so after a few months its likely it will need drying. but as John Galt said it depends how dry or otherwise your atmosphere is.