Is it incorrect to use Fisher Projections for carbons that don't have stereocenters?
According to IUPAC it is not incorrect for using it to depict carbohydrates being stereogenic or not. However here is an excerpt from the link listed below:


  The use of
  Fischer projections in non-carbohydrates is problematic. Any drawings
  of this type are necessarily ambiguous since it is unclear whether the
  author of the drawing intended to make a Fischer projection or simply
  happened to draw a diagram in that orientation. Molecules depicted in
  the style of Fischer projections are likely to be recognized as such
  when they contain at least two acyclic stereo- genic centers oriented
  vertically, although such interpretation is not guaranteed. Molecules
  containing a single stereogenic center pose even greater problems
  since it is often convenient to separate the four ligands by equal
  angles whether or not a single stereoisomer is intended. Accordingly,
  although Fischer projections of carbohydrates are acceptable, similar
  depictions of non-carbohydrates should be used only when the viewer of
  the diagram is certain to understand that the diagram was intended to
  represent a Fischer projection. Fischer projections should be avoided
  where ambiguity is possible, including when used in computer
  applications.




See more here (page 37-38):

http://www.iupac.org/publications/pac/2006/pdf/7810x1897.pdf