You'll find the main problem with running low pressures, espeshially if your heavy or riding aggresively, is pinch flats.
Hit a large rock with 'only' 40/50 psi and narrow cx tyres and there's a very good chance you will pinch flat. However, the good news is cx course don't tend to have rock gardens, just soft grass/ mud. In which case you can probably get away with running the tyres as low as 30psi. Any lower than this and the tyres wont roll right or handle right!
You need to experiment and find what works for you.
For a fast course on hard ground pump the tyres up a bit. For the mud let them down a bit.