that being said, probably a good idea but we don't use one. used to, but couldn't be bothered with buying the replacement filter and removed it. doesn't make the water taste drastically different (in our area, anyway) but theoretically blocks out lots to nasty stuff.
TBF, the filters probably last longer than the manufacturer suggestions (2-3 months, depending on usage and how hard the water is). you can probably get away with replacing the filter two or three times a year.