The denominations follow the format of the lettering of the object. Of course, there are different denominations with the same value.
Recently, I have noticed the case of Portugal, that have coins of 2.50 Euros with three different denominations (value 2.5):
The FAQ about denominations https://en.numista.com/help/add-or-modify-a-denomination-in-the-catalogue-206.html says nothing, but the FAQ about titles https://en.numista.com/help/coin-name-142.html says:
Non-integer values should be included in the title as they appear on the coin: as fractions, decimal numbers, or currency subunits. For fractional values, unicode fractions (½, ¼, etc.) or the special fraction slash should be used (⁄ U+2044).
Taking into account that an enhancement is pending in Numista that will display the denomination near the title, without to need of write it in the title https://en.numista.com/forum/topic164556.html I think the denomination should follow the same rule (to ensure its automatic application in titles, later).
Wanted & swap list (euro coins & world coins, exonumia and banknotes circulated)
https://goo.gl/AQjfKp - I have euro & world CC coins for swap.