It depends on the scheme I'm running.. and the coverage.. I like to run a lot of Cover 3.. I would advise to stay back about 12-15 yds from the line of scrimmage because there will seemingly always be a guy on a streak that goes directly into your zone.. I usually stay back deep because I've learned to not give up big plays.. I'll give up maybe 10 yd gains.. but not TDs..
If I'm running a 46 Normal scheme, I usually put the faster player at SS and blitz him into the right B-gap.. then I user the other person.. I have confidence in my user skills that I can make plays with a crappy player.. it does help to have a faster/taller player, but if I can put the faster/taller player as a blitzer to make plays shorter.. I will.