Anyone can call anything whatever they want of course, but doesn't it make sense for us all to have a similar definition for stuff? Don't we all want to agree somewhat on what 'mint' means?
I've been collecting for close to 10 years now and 'grails' always referred to the super rare super popular and valuable stuff. Things that you really have to look for and wont' come cheap unless you luck out at a garage sale. Just that common game that I just happen to like and want, well, that's just a want list game. Maybe I want a minty pac man really bad, but i wouldn't give it 'grail' status because that's already kinda a well known and accepted term. Why not come up with a different word to say the same thing?