Your portfolio should be sample enough for a client to choose you. My old boss would always have me provide samples when we submitted bids for jobs, and on more than one occasion we were declined the job but later we saw things that were VERY close to the ideas and concepts we provided. So my advice is to never do something like this (which is referred to in the industry as working on-spec). Think of the time you'll spend doing the samples...they aren't going to compensate you for that time (unless you get the job, then you can include it in your bill). I don't know about you, but my time sure as hell isn't free.