No. It's never reasonable to prevent a record being made of the actions of a police officer.

In the case of undercover officers, it is reasonable in some cases to to hide the fact they are police officers.

I believe there are cases in which police may seek and should probably be granted the ability to obscure their identities, while still acting as uniformed officers, for specific operations. The Mexican police who took down a major drug lord recently all wore balaclavas and helmets in public, for this reason. This needs to be justified on a case by case basis or you will get abuse (like the cop who attacked Ian Tomlinson).

But in every case, their actions should be legal and legitimate. They should having nothing to hide.