That's been my experience as well - good germination rates with any water source, within reason, regardless of pH or presence of chlorine.
On the other hand, 2011 was the wettest Spring on record here. I collected hundreds of gallons of rainwater and used that water exclusively for raising my seedlings (several thousand).
They were the healthiest seedlings I've ever raised, and I received a lot of positive feedback from customers about the health and production from those plants.
The low levels of carbonic acid and nitric acids (particularly associated with thunderstorms) in unpolluted rainwater result in a pH of around 5.6.
Where I live, the native soil pH is around 7.6 and culinary water has high levels of dissolved calcium ions. So around here, I think it's worth the effort to use rainwater instead of culinary water to help reduce pH in the rhizosphere and thereby improve uptake rates of cations by plant roots. This is in addition to benefits of dissolved nitrogenous molecules in rainwater.
This is anecdotal evidence, of course, as I have neither conducted a controlled study nor taken the time to see if others have.
In areas where soil pH is lower, and particularly where acid rain is an issue (such as the northeastern U.S.), I would assume that culinary water, particularly if derived from groundwater, might very well be superior to rainwater for raising tomatoes and most other garden veggies.
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