Similar to Barrett, I deal with the post-ride low BP issues by anticipating the light headedness. I have no medical background, but my assumption has always been that after a long/hard ride, the capillaries and arteries are fully opened up and the blood is warmer/thinner, so when I stand up there is less fluid resistance and the blood simply runs away from my head faster. Several hours later I am back to normal.
If it became an issue at other times, like first thing in the morning or not after a ride, I would probably see a doctor and try to do something about it. My BP tends to run to the low normal end of the range, maybe ~ 108/58, so the riding just aggravates the situation.
What are your BP numbers? Do you have access to a BP machine where you can check it over several days to see where it is running?