You're right; when the object ball is close to the rail you have a slightly bigger target (because of the "mirror" effect of the rail) and the center of this "effective target" is slightly closer to the rail than the center of the pocket opening is - so you can "cheat" your aim a little and get a slightly bigger margin of error.
I don't know how many players (if any) actually do that on purpose. It's most often used when there's another object ball close to the rail blocking the path to the center of the pocket opening.
pj
chgo