I tend to be slightly toes down unless I concentrate on it a bit. I tried forcing myself to go heel down for a while some time ago, but long-term it didn't work well for me. I noticed some slight immediate power gains but ultimately decided it was because I was pushing harder while concentrating on it. I started to have some achilles soreness earlier in the season and noticed if I dropped my heal a little more it went away, so I lowered my saddle a few mm and that helped, but that makes me closer to horizontal, nowhere near heel down.