In reply to vinayks:
If you make the signal 0, then the with expression is alway true and behaves the same as if it was not there. You cannot dynamically change the existance of a bin once the covergroup has been constructed. The iff expression dynamically enables sampling of the bin; it is always considered for coverage.
If you could explain what coverage you are trying to model, it would make it possible to suggest the best way to code it.