The battery is forcing a difference of potential between the - terminal and the + terminal. The chassis (- terminal) is somewhat isolated from the Earth because of the tyres so theoretically the potentials are different however as shown on the attached simplified equivalent schematic, there is nothing in particular forcing a voltage across your body (you will simply put the - terminal potential to a potential closer to Earth after a small equalisation current has settled).
simulate this circuit – Schematic created using CircuitLab
Even if you touched by accident the + terminal, you're right in thinking that what matters is the current going through your heart (there is also the duration of that current but nevermind that for now), however as I said in another thread that current depends on resistance for a fixed voltage. Your body resistance depends on conditions of contact and your humidity, so that's only when you're soaked wet that it would start to get dangerous (60mA with a foot immersed); however engineers never trust users and the + terminal is always covered (rarely the -, at least not on any of my cars).
Edit: [Disclaimer: Warning, the following (and above) is for information only and should not hold me responsible for any harm caused to you or others] When disconnecting the battery before holidays, I always pull the - plug bare handed... And I'm still here to write these lines.