I believe the last word I remember seeing on this topic to come down from Toady was last year ..
From thread: "The chance of failing to treat a shot as practice depends only on skill."
Thus, even unarmed, the most dangerous dwarf in the barracks is the dabbling wrestler with a couple strength modifiers. A talented wrestler will succeed in making practice shots, training skill without harming their sparring partner. Dabbling wrestlers are more prone to fail the practice shot test and end up battering spines and kidneys. Luck also has alot to do with it, any shot can fail to be treated as practice and if it slips past armor and bruises someone's brain, then thats it.
Thus, the safest way to train, in my experience, is to only recruit inexperienced dwarves, and let their XP gain as their combat skills are gained. No legendary engravers/dabbling hammerdwarves. That, and waiting for a more comprehensive healing code.