Whether or not you need
to use a balun depends on how consistent you want the
pattern and performance to be with changes in feedline length, etc.
If it doesn't matter that sometimes radiation from the coax shifts the pattern and
fills in nulls, or that noise from your house might get picked up on the feedline,
then you don't need one.
If you do decide to use one, G3TXQ has some excellent advice about the effectiveness
of various types of coax chokes here:http://www.karinya.net/g3txq/chokes/