usepackage{xspace}
newcommand{abc}{textsc{abc}xspace}

usepackage{xspace}[2006/05/08]
makeatletter
% usually check@icr is @empty and therefore
% not appropriate for xspaceaddexceptions.
begingroup
  text@commandrelax
  globalletxspace@check@icrcheck@icr
endgroup
xspaceaddexceptions{xspace@check@icr}
makeatother

defabc/{textsc{abc}}

Now use abc/ in a sentence, and it won't eat up the trailing white space.

Now use abc incorrectly.