I've got 2 66s. One of 'em's an old war horse, carried a lot anyway, and I suspect shot a lot. I just got it not long ago. It's been fired enough that the pachmayr grip is worn on the back side where it recoils into the hand, the little rubber checkering worn down. Yet, the gun is quite tight, hardly any end shake and very little slop anyways else, less than .001" of end shake, barely noticable. I had a lot more looseness in a Security Six I traded after 10 years of shooting.
My 4" 66 hasn't been fired much, is a newer gun with the newer transfer bar lock up. It's like new, no end shake, very little wear. I shoot mostly 38s in the guns, but do fire hot .357s out of 'em on occasion. I consider 'em tougher than any J frame in .357 and they seem to be as well built as the M19 I had which was fairly tight when I traded it, though the round count with magnums was kept low on that one, too, in difference to the K frame.
L frames and GP100s are a lot stronger and can sustain an large round count of heavy .357 loads much better than the K frames, Smith or Taurus. The K size guns are lighter, easier to carry, and I prefer them, but they're not as strong and able to hold up to heavy diets of heavy loads. They are not, however, the fragile POSs some make 'em out to be, my older 3" gun is evidence of that! While the Taurus might be less strong than a M19, I fail to see it. And, while everyone likes to talk about how strong the Security Six was, I didn't really see that either. I put a lot of heavy loads through mine and it was pretty loose when I traded it. The GP100's design is an improvement in strength, but the old Security Six was a lot easier to carry and I liked the gun for its size. If I'm going to go blast away with a lot of heavy loads, I'll take my Blackhawk. The Blackhawk can take it! I have the Taurus for lots of carry and not much magnum shooting. I'll practice with my 158 grain load using 5 grains of Unique. It gives a bit more of a bang than wadcutters without being tough on the gun.
End shake can be repaired rather cheaply with shims to take out the slack. If the shake starts to get out of hand, repair it before it gets too bad. The more end shake a gun has, the faster it will wear.