I was in the service industry for 7 years before flying (bars/restaurants) I pride myself on tipping well, full knowing how the industry is. I never leave less than 20% on my bill unless the service was really bad.
BUT, that being said, I had the same conundrum, being that I made so little as a f/o. and thinking about the following I decided to not tip. Follow me here..
1. We fly a large vehicle. They drive a large vehicle, We're highly trained, they are not, we generally give good service, many times they do not, and WE are responsible for many more people than any van driver, yet we don't get tipped. Why? Because it's OUR JOB. It's their JOB as well.

2. When I go to and from the airport, in uniform, I consider myself on company time. That means I am NOT CHOOSING to be at the hotel. I have to be. They are providing transportation as required in the contract. Therefore, the company should have negotiated in the contract to take care of the drivers, it's NOT our responsibility. When I am in my own clothes, asking for a ride to the bar/restaurant, I always leave a tip for the driver, because it is on MY time, and they are doing ME a service.

3. I can't afford it. The $300 a year in tips (regional flying schedules - obviously not international) is a nice chunk of money to someone making 20-25k/year pre-tax. Maybe if I was a captain, making 100k+ a year, I wouldn't hesitate, just to "pass-it-on". But sadly, thats not the case.