I've read some stories about people damaging their revolvers by shooting hot loads with light bullets. Is this a big concern with a 686? Are 125 grain 357 factory loads OK? I'm guessing that most of the loads that will damage a 686 are homemade stuff but I just want to make sure. Thanks

As long as your not HOMEROLLING HOTRODS most any 357 round is 686 friendly.

No. The 686 is an L frame, not a K frame.
Blast away with anything that meets SAAMI specs.

The 686 is a pretty healthy revolver. I doubt there's any ammo made (except, SAAMI++ reloads) that would do it any harm.

Light bullets causing damage to the gun is what inspired the creation of the 686. I had a 66 once, and I sold it to fund a 686. The odds are low that I would have damaged that 66, but when a gun says "357 magnum" on the barrel, you should be able to shoot any commercial ammo in it, and as many rounds as you want. S&W advised me to "stay away from 110 and 125 grain ammo, and keep the ratio of .357 to .38 to about 1 .357 to 9 .38".

+1 on what they said as you can shoot any factory ammo in it. If you reload just stay with in the reloading book recommended power charge and you will be fine.