How much numbness is too much numbness?

No.. you angle it VERY VERY slightly up. It depends on the shape of your arse and your position on the bike but for some people it can help rock their hips back and relieve some pressure from the front of their undercarriage.

My saddle is flat horizontal and I get mild left hip pain after any ride of considerable length recently, i will try angling it backward to see if this reduces the pain i'm getting. Cheers!