There are some slight NMR advantages to going to lower salt concentrations. There are severe technical disadvantages to increasing the ionic strength significantly. At very low salt, there are problems of trace contamination, and serious modification of ionic strength when titrating buffers. Less than 10 mM is probably not generally a good idea. In general, buffers should be NMR invisible, i.e. no hydrogen atoms.
Many deuterated buffers are available.
3.3.4 Other solvent issues. Probably need deutero-dtt. Light scattering or NMR detected diffusion coefficient can tell you if you cannot get homogenous monomer. This happens very frequently. Typical contaminants include glycerol, dtt, tris, methanol, other alcohols.