It's never occurred to me to tip because the interaction is usually so fleeting even if the usher walks me down the aisle. And I usually decline assistance since I know the big houses here in SF pretty well and can easily find my seat.
There was one time I tipped though. I flew down to San Diego for the day to see the Hunchback premier at La Jolla Playhouse. If you're unfamiliar, LJP is tucked away on the sprawling UC San Diego campus and often not an easy place to get cabs. I had to dash to the airport after the show. I asked the usher how difficult is it to for cabs. She casually said "Don't worry about it, I'll call you one." I gave her my name and seat number -- she didn't write anything down -- and she hurried off to help the next patron. While exiting I wondered how difficult this was going to be, knowing this lais-sez faire usher was probably sitting in the office with her feet up.
Hardly! She was waiting at the top of the aisle, called me by name (formal no less: "Mr. H_____"
, and personally escorted me out the door through the crowd and down the sidewalk where a black sedan was waiting. She wished me a pleasant flight home, genuinely invited me to return for other productions, and sent me on my way. I've never felt more like a rock star!
Her efficiency and respect for my timing made the whole thing happen so quick, it didn't even occur to me until I was tipping the driver at the airport that I should have tipped the usher. Next day I sent a letter lauding her -- and the production, which despite the obvious and generally agreed-upon flaws, I loved -- and enclosed $5. I hope she received it.
And yes, I made my flight with lots of time to spare thanks to her.