Tomatoes are very low in carb, I think it's 3g per 100g so they should make little difference. But you need to watch processed tomatoes like tinned ones or tomato sauce etc. as sugar is often added to counter the acidity.
Jen, I've only ever taken the values off the package, they give both dry and cooked carb value. But as you say everyone cooks for different lengths so that could be making the difference. I do cook the pasta for a few minutes longer than recommended on the packaging so that's probably where the problem has been.