It very much depends on the objective of the display and the institution concerned and also the target audience for the display.
Sometimes the coins are displayed with and adjacent other antiquities
of the same period, often one or a few coins inserted among other antiquities
. These are usually examples of very high grade
and artistic merit
and appear to be on display as a compliment to the other items and thus give a more complete
overview of the period e.g. some of the displays in the Metropolitan. Understanding context is the objective in such cases.
At the other other extreme the coins are displayed row on row, cabinet on cabinet, as part
of a massive collection
on display to give an understanding of the development of coinage, or the coinage of a particular place, or era e.g. Athens
Numismatic Museum, or the coin display rooms of the British Museum. In this case the the best available examples of a type
available to the Museum are usually on display but grade or artistic merit
are not the paramount determinant, rather it is the comprehensive understanding and representation of a specific coinage that is sought to be conveyed. This sort of display is directed very much to the person with a strong numismatic interest, or research interest, rather than the general visitor to a museum.