It's a case by case thing.
I don't recall many posting that they are making extra money, probably for the very reasons you mention..

The SS site does say somewhere what the extra earning limit is-

[Two things can cause us to decide that you are no longer disabled and to stop your benefits.

Your disability benefits will stop if you work at a level we consider "substantial."

In 2015, average earnings of $1,090 or more per month ($1,820 or more per month if you are blind) are usually considered substantial.

Your disability benefits also will stop if we decide that your medical condition has improved to the point that you are no longer disabled.

You are responsible for promptly reporting any improvement in your condition, if you return to work, and certain other events as long as you are receiving disability benefits. Page 11 of the booklet we send you when your application is approved explains what you need to report to us. ]
from this page-
http://www.ssa.gov/planners/disability/dwork2.html