I have a susbscription to Ancestry.com, that's how I got my info. I don't think there is a site that you can look at a copy of the actual record of WWI registrations for free. I could e-mail you the image if you like. As for census records, here is a link to free census records:
Census Finder - Free Census Records Online Sometimes the information in census records was not given by the person who it pertained to, it may have been give to the census taker by a wife or child who may have been unsure of the information. Good Luck!