The mirrors on my Sprint have a detent built into the stalk so if they do get bumped it's simply a matter of moving them back to the detented position.
As for placement, I like to be able to see just the edges of my arms. That gives me a reference point when trying to decide where an object in the mirror actually is. I've tried pointing them right at the blind spot but just can trust what I see since I have no point of ref. to compare it to.
Having said that, I've done this
to several of my bikes...