Dimezone-S at Kodak was called HMMP or Hydroxy Methyl Methyl Phenidone. It has the best balance between keeping and being a powerful developing agent.

The Bisulfite anion HSO3- decomposes rapidly to SO3-- and then scavenges oxygen to form SO4-- or sulfate anion.

The metabisulfite anion is much more powerful a reducing agent, and one reference I have seen states that it can catch fire during the grinding process to break up lumps. It undergoes exactly the same reactions as above but the initial breakdown from metabisulfite to sulfite or bisulfite (pH dependant) is very quick and strong. This is Na2S2O5 or the S2O5-- anion. The Metabisulfite gives one mole of SO2 and one mole of SO3 upon decomposition in water, both of which end up making HSO3- or the equivalent. It is acidic in water.

A second reference gives no comment about Sodium Metabisulfite catching fire. However my references do ascribe this to the Potassium salt

So, I would tend to say take precautions with the Potassium salt.