Just to clarify (and adding my minuscule knowledge of batteries) most new batteries are sold as "maintenance free", but that does necessarily mean that they don't require venting as there are a few types that are internally completely different. When I think of a vented battery, I think of the "wet cell lead acid battery" which (I believe) is still the most common, but even the newer VRLA batteries do/can off-gas hydrogen, but there is a sub-category of AGM (which I have no first hand experience with) which uses an internal mat to absorb gassing.
Bottom line, if there is a hole, plug or bung at the top to accept a vent tube, plug one in. If there isn't, don't go drilling holes so you can plug one in.
