Actually, you don't need any medical unless your are exercising the privileges of your pilot's certificate FAR 61.3(c)(1) and FAR 61.23(b)(5). In other words, you are ACTing as PIC. Remember that an instructor's certificate is NOT a pilot's certificate. So as long as the instructor doesn't have to ACT as PIC then no medical is required. For example, a student working on their commercial may be able to ACT as PIC for every flight lesson and therefore the instructor doesn't have to have a valid medical. On the other hand, a private pilot student can't ACT as PIC unless they are solo, therefore, the instructor must ACT as PIC during training, hence the instructor must have at least a valid third class medical.

As for the checkride, again, if the examiner agrees to ACT as PIC (not the usual case) then you don't need a medical at all. However, normally the applicant for any checkride (even the private checkride) acts as the PIC. This requires the applicant to have a current third class medical. See FAR 61.47(b) and 61.39(a)(4). This is true for any checkride (other than those ratings that don't require a medical---like a glider rating). Both the commercial, ATP as well as instructor checkrides only require a third class medical to take the checkride if the applicant ACTs as the PIC.