Aim of a 'smooth' histogram
When drawing histograms, we usually aim for smoothness in the outline of the histogram rectangles.
Ages of patients admitted to cardiac unit
The width of the histogram classes should be chosen to give as smooth an outline as possible.
There is additional flexibility in chosing where the first class starts — it does not need to start on a multiple of the class width — but this has a much smaller effect on histogram smoothness
Long-distance telephone calls
Choosing histogram classes to get a 'smooth picture' makes its 'message' clearer when you include it in reports. However the choice of histogram classes, within reason, should not affect your conclusions about the data.
If your conclusions from what a histogram tells you about a data set depend on the choice of histogram classes, you are over-interpreting its shape.