For about two quid and five minutes work Id rather be safe than sorry, thats a rather uninformed statement given that you know nothing about the battery installation in question, it could be in a sealed box for all you know which would be full of gas in a couple of hours charging, I have seen a hydrogen air mix explode and its seriously destructive even in small quantities. If the battery is under the bonnet or in a well ventilated area then I agree there is no need to vent but if its contained in a tight space then you could get a buildup.wouldnt bother doing that my opinion its a waste of time
deep cylce batteries are semi sealed. a proper leisure battery should not give off much gas at all.
I have not bothered. its only a really knackered old battery that will out gas loads.