In the engineering world, there is a saying. If something is going to break, it's usually going to happen near the beginning of its life, or near the end of its life.

In my experience, I've had wheels crap out within days of purchase, and after years of use. Never in between. I agree with a custom 36 spoke rear wheel. 40 is probably overkill. It all depends on your comfort level with risk. If you have someone who can pick you up if the wheel fails, stick with the stock wheel and replace if it happens. If you're a loner like me, go custom and don't risk it. I made a 7 mile walk of shame because of a crappy stock wheel that died 3 days after I bought it. Would have been 17 miles except the bike shop on the way home offered me a ride. I nearly quit cycling altogether.