I'm not saying attacks should never hit a character, if someone's specifically being a Marty Stu and never getting hit when they could have afforded to take a hit for dramatic tension or such and they just decide "My character's too awesome to ever get hit" and godmodes through everything, sure auto-hit them a little. But moderation's still key in everything and you have to work with the player and know what kind of damage crosses over from "build tension and make things more exciting/dangerous" to "potentially permanently crippling or killing someone's character" and stressing them out over it maybe.
I think most of the time at least, you should trust your players to react appropriately and decide how/if their characters get hit.
I think most of the time at least, you should trust your players to react appropriately and decide how/if their characters get hit.