Calculating based on the time the mail is sent is the traditional way of handling DDGs. I doubt there are metrics but the reality is that the old way is probably more accurate of membership, but on the other hand, calculating nightly is probably not a big deal for most orgs - especially if they know that membership will not be accurate for 24 hours.
I would point out that you can still update a DDG manually in Exchange Online. From what I have seen, you dont need to wait 24 hours, you can do this after any change