You can get whole manifolds with check valves for less than the price of the check valves. e.g., Ritebrew.com
But the brass manifold you have is better than those.
Do you need them? Yes and no.
I'd like to have at least one check valve between the regulator and the manifold, to protect the regulator in case of a beer backup. A long line (6') from your regulator to your manifold provides a little security buffer too.
I went with MFL fittings to be able the take the gas lines apart easily for cleaning, in case they need it. Happened 2x.
Agreed, an overfilled keg may push beer into the line if the pressure drops higher up. Otherwise it's a rare occurence.