I don't even turn the Manual Grab option on at all because then I'd be sacrificing her more than I do now !
The auto-grab takes my mind off anytime she's on the edge of the platform and when an enemy's shooting at her or something to force her off, she'd grab onto the ledge immediately.
If I had the option on, I know I'd be forgetful about her being able to grab onto the ledge to prevent herself from falling.
Seriously, if I recall correctly, the Action button in TR3 had her do several tasks such as grab onto the ledges, pull levers, grab and hold movable objects and some others I think. Now that we have a separate Grab button option to enable, it feels awkward to me. I was trying to get used to it when it was on but just couldn't. If the Interact button would also have her grab, then maybe I'd feel more comfortable. Then again, maybe not. I don't know.
The last paragraph was typed before I read Boots' post. So if I sound like a broken record.... I sound like a broken record.