I can only say that I have strung many a frame and I have never snapped a string because of a grommet.
I typically check each racquet over moderately close when I receive them.
Now I check them over very close.
One time after checking over a racquet, I started stringing it with all VS gut, and pulling a main string caused a slight crack in the grommet to open up and become more visable, and sure enough once that small crack opened and the string rubbed against the frame the string snapped.
If this happened to you, you will check them over much more closely afterwords, as that was an expensive error. Bad grommets or grommets that are not seated properly, or not the proper grommet to protect the strings will snap strings, and if not at stringing then shortly later.The grommets are there to protect the strings, and if they are not in good condition or not proper length to do their job you will have problems.
BTW, that racquet was given to me to string with that same string snapped at that grommet, but it looked fine , but the small crack in the grommet was only really visable once tension was applied to it, and once that happened it was too late to do anything. I now look over all grommets when the racquets come to me with the strings in place, and also checked over once the strings are removed.