It may be a very good idea, as most of the serious injuries, at least according to the Safety At Sea seminar I attended last year, were either boom or MOB related. Probably want to go with something like a rock-climbing or hockey helmet, rather than a cycling one...
The other option is having the boom raised a bit on your boat... so that the area where contact between the head and the boom is reduced... On my boat, if you're in the cockpit, it is almost impossible to get hit by the boom, unless you're head is above the bimini.
Also, installing a boom brake can reduce the speed and force the boom can exert when it moves accidentally... making it much less likely to injure you.
The problem with a helmet on a boat, is that they can be hot and uncomfortable, and it is easy to forget to wear them... and not everyone aboard will have one that fits them. IMHO, it is generally better to make the boat safer by reducing the risks, than accepting them and putting a helmet on as part of accepting them.
Even though the boom on my boat isn't much of a danger to people in the cockpit, it still represents a bit of a danger to people working on the cabintop, so I have installed a Dutchman boom brake on my boat, and would highly recommend it to anyone. It reduces the danger of the boom in an accidental tack or gybe, and also helps protect the rig from the shock loads of an accidental gybe.