I only noticed it when I was actively seeking the refresh proc when attacking dummies. As others have said, the 15% proc rate is too low too care about it. It's a nice cherry on top of the sundae that is Bane.
However, after you have casted your DoTs so many times that you can do them in your sleep, you begin to instinctively know when to refresh them manually and if it seems like your DoTs are lasting longer than you're used to, it is likely the refresh proc'd.
