Despite what people say about the newer the better, I see no evidence to suggest that this is the case. It can happen to any age bike at any time of its lifespan.
Many have reported no problems upto 15K miles, and then suddenly 2 or 3 need changing. Other have problems after only a few thousand.
The key appears to be that the bike need to be warmed up properly. It has been shown that it can take upto 90 seonds for the oil to be delivered to the rocker surfaces on start-up, so if you have a habbit of getting on the bike and gunning it from the off, then the rockers get a bashing before fully lubed up. Not good.
To answer your question, yes, people do buy the bike and worry about it if and when. I would suggest that if it is a big deal to you, then perhaps you should be lloking at another manufacturer. The fact is you could buy a bike, have it checked and its fine. 2000 miles later it could need X amount of new rockers...you just never know.
But you'll also find that most peps on here don't really give a toss about that, and ride the bike of their dreams and put up with the 'problems' that may come with it