I don't think it's so much a numb perinium that's a problem. It's when pressure on the perinium area causes your penis to go numb on you, for whichever reason, and that's not good. I usually reach down and grab my goodies every now and then on longer rides just to make sure there's no numbness. If there is some numbness, or even just a hint of loss of feeling, I stand, or shift to one side of the saddle a little, or adjust my shorts.
I've noticed that I can't tell I'm going numb unless I touch it. There were a couple times I rode quite a while, then adjust my shorts and noticed that I just about lost all feeling down there, and that was just a bit scary. That's why I do the regular "grab it" check when I'm riding.