It's important to install an antisiphon valve at the tank/hose connection. It is a simple device that won't allow the passage of fuel unless there is a negative pressure on the pump side. In other words, when the pump is providing suction, the valve will allow flow. If the line breaks or is torn loose from the valve it will keep fuel from flowing out. I did some research and did not find any USCG or Texas state requirement for them, but I think they are required on new boats by AYBC standards. It's a cheap safety device, no reason to exclude it.
I would put the shutoff valve in just for convenience during maintenance. Shut it off when you change the fuel filter and it will hold the siphon in the lines and pickup tubes and your engine will start faster when you're done. It also makes it easier at the end of the season to run the carb dry if that's one of your storage procedures.
As far as restrictions go, unless you are running a huge HP engine, a 1/2" line is more than adequate and the pressure drop across the valve will have no effect on performance.
Don't forget to use all stainless steel hose clamps. The ones you get at the auto parts store will rust (at least here in Texas they will).
Besides, when you "told him that all the boats I have owned or worked on had a fuel shutoff valve." that's a good enough answer. Don't let other people's opinions dictate how you do things. If he was right, all of those boats would not have had a shutoff.
http://www.iboats.com/products/7/239...on_valves.html