Depends on what you mean by "worry." Do you mean should you worry about losing your investment by ending up with a cracked, unridable frame? Can't help you with that one.

Do you mean should you worry about the frame disintegrating out from under you on a ride? That I can address - it is highly unlikely. Some frame materials fail catastrophically Right Now if they fail. Steel ain't like that. If it fails, it fails sloooooowly. What that means is that you will most likely notice a frame crack from handling issues well before the frame collapses or otherwise launches you.

To all the research and written material on this subject, I will add one piece of personal-experience anecdotal evidence. I had a Bridgestone RB-1 develop a crack that involved the seat tube, the down tube, and the bottom bracket lug. (My theory is it started some years earlier from a crash I had, but I'll never know for sure.) On a ride, I looked down to see my chainwheel appear to be dancing a samba. I stopped, looked, and saw the crack. Okay, I know I should have seen it earlier through post-ride inspections, but the point is that the crack clearly was not new had spread - the steel was literally tearing - but it was doing so in such a way as to give me plenty of warning before it causing a crash. So even if your Miyata develops a crack, odds are you will have plenty of warning before it gives way completely.