Yes. I’ve rarely recommended that folks change their name servers, but a few have anyway. Adding the records to Digital Ocean’s name servers doesn’t hurt (since nothing will be looking there) and it does keep anyone else from using Digital Ocean’s name servers for their domain.
My biggest concern is that they’ll switch to Digital Ocean and not move their other records and hose the rest of their domain.
As I’m sure you remember, I’m often confronted with people who have delegated their DNS to some third party hosting service and can’t figure out how to change anything. The worst was a company that I had to email to get them to modify the DNS manually.
I think that’s true, but I saw one article (which I won’t try to find) that indicated that the resolution times for Digital Ocean were really good.
Reducing TTL seems to be a big win only if you expect DNS to be changing often. . .