Sure, but say 6000 mg, don't say 6k mg.
That'd be ideal, yes, but I'll take 6k mg over 6g when mg are the working units every time. The k makes it much more explicit that there was a shift in magnitude than just silently converting between units.
It has nothing to do with being uncomfortable with working with SI units and everything to do with having a consistent working environment for me.
For example, If I were drawing something up in CAD and needed a particular face to be 120mm, I'm completely aware that could be represented as 12cm, but I want the software to label that as 120mm so that it's consistent with e.g., a 23mm face next to it.
Your working units for sodium in food consumption are milligrams. Labeling the final sum the same way is a good thing.