informal rush

I'm just wondering how the COB process works. I know some houses will do Informal spring rush, but is that only the bottom tier houses that haven't met their quota? I really want to be able to COB but I just don't know what steps to take.. Should I email houses now and ask if they will be COB'ing at any time this year or do I have to wait till the spring and have to have friends in houses that can put in a good word?