Is it necessary to setup a project under a client if you know there will only be one project associated with that client?

If your new client is only going to have one project, is it necessary to set up both?

No.  Simply set up your new client via '+ New' - 'Client' and start working.  When adding new a Item, Proposal, etc., simply choose the 'Client' from the 'Client / Project' drop down.