When doing a ECU job, you know it'll eventually need to be paired to a CAS. So, when I do such a job I always insist that faulty DME is delivered to me with CAS and Key. That way, when doing ECU work you have the intended CAS available to you. That way, you know the correct programming and coding is done before pulling ISN from ECU in order to align.
If someone asks you to just get ISN off DME, you must remember that sometimes DME software is too old and must be updated for Autohex to be able to pull ISN. If you don't have the CAS available then you won't know what software version is correct to update the DME. The point is, if you have the intended CAS available then the risk of problems down the line becomes a lot less.