Putting a signature in your email is worthwhile, as this is beneficial to the readers of your emails whom can go direct to your website(s) and view the services or products you are offering. From an search engine optimisation point of view - it has no bearing whatsoever, as the email signature is not going to be indexed by Google, Yahoo etc. For brand awareness I wouldn't hesitate to recommend adding a signature to your email, perhaps even add special offers or focus on a particular product or service, to attract visitors to a particular page that you want to promote more.