Think of it this way...if you leave for work about 7:30, that means you're toweling off  7ish.  So, 5 hours later you're not exactly fresh.  Not to say that you and the construction worker to your right at the red light are in the same aromatic place, but why not just pop in for 2 minutes, soap up the smelly places and put her mind at ease.  I bet you'll find it to be a good tradeoff.

Some ladies are more of a stickler to the shower before rule than others.  BUT, it only takes 2 minutes to lather up and be as clean as a whistle.

Now, you know you want her to take care of the whistle...so you definitely want that to be clean.   Even if you shower in the morning, I am sure that you would have used the potty etc before lunch.  Two minutes will give you a much better session and the attention that ALL of you deserves

I shower before each session upon arrival. It sends a very nice message to the Ladies and I find my mileage improves during the session.  I read a lot of reviews where the reviewer states CBJ where I know I was fortunate to get BBBJ with the same Lady. I think it is a direct result of the message you send. Fresh breath and clean for head to toe.
Happy hunting.

Thanks to all who replied, and good point I hadn't thought of Cowboytoo about CMJ vs. BBBJ.  I'm convinced now and will be showering my next time.  But would y'all suggest I only worry about showering if offered?  Or is the consensus that I ask for it even if it's not mentioned?  After one semi-messy session I asked to shower afterwards and the provider said ok but didn't seem thrilled, so I was wondering if it was then a towel-shortage issue.

Again, thanks for everyone's replies!

really though, you can usually tell if the provider is just offering it as an option (or even an icebreaker) or if it's part of the provider's sense of security.  Do what your gut is telling you...