I get ready before the date but understand completely if she would like me to
shower when I arrive.

In the summer, a lot of sweating can happen on your ride to her place so I still definitely recommend a quick 5 minutes to wash your undercarriage. I'd never want a slight hygiene issue to keep my tongue from going where it wants.  

But it doesn't take me five or ten minutes.  I shower before leaving home so the one at her in call is a two or three minute affair...unless of course she gets in there with me!

If someone offers you mouthwash and a shower, don't debate them.