Mike Schulz
"In our area it's a must specially if its clay soil. Sandy soils seem less of a problem. I tell my clients to close there vents in the summer (humid) and open the rest of the time. Freezing is not a issue most of the time."

Close the vents in summer!?! That’s when they are needed the most!
My opinion of the crawlspace is the exact same as the attic. You can't vent or seal it half assed either way.
If the crawl is to be sealed, (closed vents) than it must also be insulated and conditioned just like the rest of the house. The vents should not only be closed, but completely, 100% sealed. Closing a cheap plastic vent in a foundation wall during July will not keep humidity in the crawlspace down, but will only make it worse.

If the crawlspace is to be vented, (open vents) the interior should be completely sealed from the crawlspace. Any ducts in the attic or crawl (bad idea to begin with) should be well sealed and insulated to prevent the conditioned air from being released into the crawl space or being pumped into the building envelope.

Mike Schulz
"install vapor barrier except under a air handler and close vents during humid months. That typically solves most problems."

I would think a vapor barrier under the air handler close to the ground would be a good idea. What would be the problem with that?

O.K. now everyone put in your 2 nickels on the open or closed debate. I would love to hear what everyone else thinks.

P.S.
How do you fellers use that fancy quote feature?