It is true - something to do with the possibility of breathing in accumulated dust mites...I think.

But I must confess we have never bothered. The same mattress will do all 3 of my kids but my kids have all moved into big kids' beds pretty young so it's not really had a lot of wear, and has never been in storage (and has never been the scene of a major poo or vomit blowout that has gone through to the mattress).

Like someone else posted, we always use waterproof mattress protectors and I change the cot sheets at least 3 times a week because babies can be very sweaty little people, not to mention they leak a lot in the early days too!

I will be passing on the cot when we are done with it but not the mattress to make sure that whoever gets it has to buy a new mattress in accordance with the guidelines. And personally I wouldn't accept a second hand mattress from anyone else just because I don't know its "history", so to speak.