I prefer the data remains in Dataverse (Common Data Services) even if the survey itself is deleted in Customer Voice. Customer Voice I believe has a limitation of 200 surveys. For my business, I'm forecasting to hit this 200 limit in about 12 months time and will still need access to the data for surveys older than this. If survey data needs to be deleted within Dataverse then there should be a facility to initiate it's deletion. There should be a flag in Dataverse to show whether the survey has been deleted, or not, within Customer Voice.