Additional cover on reinforcing steel
It is important to recognise that reinforced concrete will generally crack; only when a crack forms does the reinforcement start to carry any appreciable load. Crack widths are limited by the design process to maintain performance and durability, with the width being controlled by the reinforcement. The crack width at the surface is related to the cover thickness.
There are situations where the cover provided is greater than the design value (for example to allow for future abrasion or due to errors in construction). Cracks taper from the surface of the concrete to the level of the steel. Increasing the cover will, approximately, increase the width of the crack pro rata, which may have aesthetic implications. However, when checking the durability provisions, it would seem reasonable to calculate crack width at the specified design cover rather than at the actual cover.
Conversely, reduced cover can lead to more effective crack control. But beware not to compromise the durability afforded by the concrete itself.