In reply to john arran:
It is going to depend on a few factors, length, exact profiles. A few examples I can think of lend themselves to different answers
I regularly do a 1 mile road loop on a business park, which rises about 15m over 400m, then drops 15m over 250m, when run clockwise. It is a good bit faster clockwise than anti-clockwise.
My club is organising a 6 mile road race on Friday, is has more than 150m of ascent, the ascent probably totals 1.5 miles total distance, descent is probably 3 miles. That is definitely slower in reverse, although the average descent gradient isn't much it has some steep sections that really get the legs, so maybe not a fair comparison.