It seems to be hit or miss. My 442 was a very early production (Sept 1993) on the original J frame Airweight. I never noticed the crack until about two months ago when I read the thread on this forum and decided to take a look. Look up the thread "The dreaded frame crack" and you'll see mine and other stories.
The good news is that S&W will stand behind the revolvers. In my case the bad news was that they don't make the non "J Magnum" frames any longer and so I couldn't get it repaired, it had to be replaced. If your 642 should crack, what they are likely to do is replace the frame and barrel and transfer the rest of your parts over to the new frame.
So, the bottom line is even if something does happen S&W will stand behind it.
So, I would say no, you didn't chose poorly because you bought a product by a company that will repair or replace your 642 if the need arises.