Is it really necessary at 100 Hours? is it as easy as the service manual illustrates?

 

As far as it being a "must". I don't think so. It is necessary if you want the engine to breath and exhaust when it should for best performance. Adjusting the valves means that they start to open and close at the correct position of the cams. You need to think that every time they close they slap the valve seats not just ease against them,they wear a pattern which means they tighten up just a little so they don't open as much as needed.
I'm no professional so this is how my Dad explained it to me. May or may not be right but has always worked for me. RIP Dad.

 

Do what dyna105 suggested and check the valves at 100 hrs.You'll probably find on the 400 that it's like the 500 and many I checked if anything were set up at a "loose" .006 at the factory. 95% percent of the time even on machines 2 to 3 years old or older all I had to do was check the clearance and most a were still spot on.Only on the really neglected machines will you probably have to do any adjustment.