But why not add a little sealant? Is the bottom surface of the windshield so flat, and the curve of the dash so exact, that a leather gasket will seal up perfectly? You can duck when water comes over the wind screen, but not when it comes under it. (Either way, though, you get soaked somewhere.)
I guess I'm of the school that I like to over do things (that is, whenever I actually do something). If building in wood, glue is great, but screws add that extra hold, and vice versa.
No one will notice the sealant, and it's an extra precaution. Although, it's all pretty pointless in a car with scant weather protection.
Just wear waterproof clothing...