"Now, Mrs. Jones knows what the job will cost and is expected to pay me when I am done. vs.

"Mrs. Jones, we are all done. It took a little longer that we had hoped, but here is the bill - 3 hrs labor x $70, etc, etc."

Mrs. Jones response "it shouldn't have take 3 hrs to change that ceiling fan. It should only have take 1.5 hrs. That bill is too high, i'm not paying for 3 hrs. I'll pay you for 1.5 hrs, etc)
"
That is the main reason we do contract, to avoid the hourly fear factor.