From what I understand, registering lots of related domains and forwarding them isn't really going to help you hugely for SEO for the original site.
Extra domains are more of value if you are going to produce unique content on each domain and let it build some value of its own. You could then link from each to your original site (but try to put the subsites on different hosting to the main site otherwise it all looks a bit too obvious).
If you do 'collect' domain names like I know many people do, and dont want to do much with them for the moment, then at least make sure they forward to your active website using a 301 redirect, not your domain providers url forwarding.