I believe it really depends on the size. I've never had a clam under 2", so I don't know if it is vital to feed them - but that's what others say. The larger/older ones pretty much live on their zoox. from the light.
I believe the larger clams will be (at the risk of sounding anthropomorphic) 'happier' if phyto is added to the tank, but I don't believe it's necessary to sustain them.
I feed DT's b/c I think it's generally good for the entire tank - clams, corals, pods, etc. I also like it b/c it is live - I'm not adding dead stuff to my tank.