This reply coming from a systems builder: It always depends on the situation, but generally basic ideas like the aforementioned example (wheels on feet) doesn't much require asking permission - now say, if you build specific animal like a moose and give it a wheeled feet and someone comes along and makes his own moose with wheeled feet then he would be proper to GIVE credit where it is due - but by no means should they have to ask you. Do you ask LEGO to use their pieces for each MOC?
While sub-themes are less common aesthetically with Bionicle, it is generally considered proper to ask before building in a specific theme. However, bad MOCs are generally considered exempt because their usually ignored. Any builder trying to become a respected member of the community and a better builder will however be expected to ask - as this keeps things civil and the community closer.
