I don't think it is, but—as usual with EC—if you don't want it there's a performance cost. The way you'd avoid it in a model where writes go into one queue/connection and reads come out somewhere else is when you'd do a write you'd have to block and wait on that write being acknowledged on the reader side (or you build in some sort of side channel to whatever your write handler is so it can tell you when a write has been accepted, if you're doing conflict resolution on the write side (some systems don't and push it off to read-time)).

But at that point you're just simulating something you probably already had before going to CQRS/ES, so you would only want to do that in very select cases. Otherwise there's really no point to the architecture...