Capable agents know within seconds after starting to play your demo, whether or not you're a voice talent they want to hear more from, and perhaps represent. Yes, seconds. Every cut on your demo must shine, but the first one will either make them want to hear more, or hit the "stop button." That's why voice actors and demo producers give considerable thought to what the first cut is going to be. And like a good radio aircheck that's used to hire deejays, each cut on your demo should showcase something else you do well. Once you've shown that you do a great soft sell commercial read, or hard sell delivery, there's no need to show that style again.
During my on air days, I wandered into my program director's office. He was looking to hire a new deejay. I saw lots of brown packages spread all over the floor that were mailed in from around the country. Each one contained a radio aircheck-a sort of "best of" clips taken from a deejay's show to allow the hiring program director to hear what the deejay sounds like on the air. I watched as he put the CD (or cassette) in his player, and then pushed the play button. Most of the time, within seconds, the aircheck was being ejected and another put in. To some folks that might seem harsh. But that's the reality of the situation.
So remember, when it comes to voice over demos, give much thought as to the order of your clips. And having a second set of ears to listen, wouldn't be a bad idea. The key is to have someone help you who actually knows the process within the voice over world. The mailman probably wouldn't be a good choice. We love our dedicated mail men and mail women. But just sayin'