I don't know how much effect this has on the rendering, but many of the symbolic icons are not *exactly* 16x16 pixels. Eg:

for f in *.svg; do echo -n "$f: "; sed -ne "s/<svg.*height='\([^']*\)'.*width='\([^']*\)'.*/\2 x \1/p" $f; done

  ...
  battery-caution-charging-symbolic.svg: 16 x 16.016466
  battery-caution-symbolic.svg: 16 x 16
  battery-empty-charging-symbolic.svg: 16 x 16.016466
  battery-empty-symbolic.svg: 16.000002 x 16
  battery-full-charged-symbolic.svg: 16 x 16.000137
  battery-full-charging-symbolic.svg: 16 x 16.016466
  battery-full-symbolic.svg: 16 x 16
  battery-good-charging-symbolic.svg: 16 x 16.016466
  battery-good-symbolic.svg: 16 x 16
  battery-low-charging-symbolic.svg: 16 x 16.016466
  battery-low-symbolic.svg: 16 x 16
  battery-missing-symbolic.svg: 16 x 16
  ...