Let us start with a very famous quota which can be applied to general tasks. Nevertheless, in the field of geometry preparation for CFD purposes has significant importance:

“Everything should be made as simple as possible, but no simpler.”
Albert Einstein

During a flow path geometry definition, one should keep in mind a simplicity of the geometry. For a simulation of a complex turbomachinery geometry, tiny details which does not significantly influence the flow should be removed from the flow path. Such details can not be captured by a reasonable amount of cells or can not be captured at all.The picture below depicts an (artificially created) example of such a detail.