With components you really shouldn't have a large separation between the woofer and the tweeter. In the same respect the tweeter in a coax would do no good being blocked by your leg.
I have Polk/Momo components up front with the tweets in the dash. I had originally planned to move the tweeter closer to the woofer but wanted to keep it stock looking. So I left it in the spot for the 3.5". It sounds good to me and that's all that matters!