I'll do it without hesitiation. I'll say something the first time like
"uhh, you're just a little close to that hitter. You might wanna back up an inch or two", and the second time,
"I've seen too many catchers hurt by a bat head moving at 100 MPH ... please be careful". I've never warned the same F2 a third time, though. Once, a catcher made me really nervous after two warnings ... during the next mid-inning offense/defense change, I walked over to the manager, pulled him aside and told him I thought F2 was setting up dangerously close to certain hitters. It's really
his job to coach his players, anyway.
I have a duty to enforce safe conditions on the field, and those things are always judgment calls. If I
hadn't said anything, I'd feel awful. If I'd said something, and the catcher were still injured by later ignoring my advice (and I'm careful to state it as
advice, not a
warning), I'll feel only slightly less awful. But at least I said my peace: I can't get on myself for failure to do my duty. It's funny: I've had no catchers bark back at me, and no coach has ever pulled me aside to gripe at me over it. I think everybody understands my motive.
You asked when you ought to let common sense kick in.
I'd say 100% of the time. Sure, I want to work a good game -- exactly by the rule book -- all the time; no coaching, be as professional as I know how to be.
But my role in ensuring that play is as safe as possible takes precedence. When somebody
does get hurt, it takes a lot off of the fact that I might have called a good game. I hate it when that happens. No matter how we might try to make conditions as safe as possible, players get hurt sometimes: bats, balls, and players moving at high speed aren't always kind to flesh and bone.
I feel for you. I guess this is how you learn when it's worth it to open your trap and pass some friendly advice on to F2. I'd bet some of the more senior folks here will probably tell you the same. Or not
But don't beat yourself up over it. I've observed that most catchers will take the advice -- for a pitch or two -- and then set up right back where they were.
Should I keep on reminding them? I'll never do it more than twice. After that, I'll discreetly let the manager know what I think; and then it's done: I've got a ball game to call -- all I can do is hope and pray that the very sad lesson being risked won't be learned on
my field.