I have personally seen many heads from all makes and models that have had some recession, but don't have any pictures of my personal experience. I can't say they were all due to unleaded fuel alone.
Here is a small thread from this board on the same topic, and it seams there is some dispute on how much unleaded fuel causes recession.

http://forums.corvetteforum.com/c3-t...s-to-fuel.html

Below is a repost from another site from a very reputable source which probably sums it up very well.

"If your Chevy Corvette (or any vehicle) was built to run on leaded fuel and hasn’t had hardened valve seats installed you probably want to think of using an anti-valve seat recession additive no matter what fuel type you use these days. However, there has been considerable talk of whether hardened valve seats are really needed. Some have found that hardened valve seats aren’t essential for normal daily driving where the engine is not subjected to extreme heat. So unless you are drag racing, pulling a heavy load constantly (i.e., a truck towing a trailer), have a turbo charger or something similar that puts a lot of RPM’s and a lot of heat (like where the exhaust starts to get red, running lean EPA-style mixtures), then there was no big problem running unleaded in an engine without hardened seats."

You need to research and make up your own mind, I tend to lean towards an ounce of prevention.....
With the cost of labor, parts machine work etc, seems like cheap insurance, and it certainly couldn't hurt to either have hardened seats installed or use some form of additive unitl they wear.

 

I did that many years ago and could find NO credible examples of valve recession on Corvettes despite repeatedly asking for examples. I came to the conclusion that it's just another 'the sky is falling' myth perpetuated by snake oil sellers that insist that 'bad things will happen' if you don't use their products.