It's an odd practice IMHO. DVI really needs about an hour to wake up, and putting it in with the rennet means it might not be fully distributed in the milk by the time coagulation sets in. Without even distribution, the cells become trapped and the pH profile would differ in the vat among areas. Washing would help mitigate this, but still not the best practice.
I could see doing it with bulk or frozen, that wouldn't be so bad. Toss it in, do a good stir and done.