I guess I just count germination as when I see for sure two little baby plant eaves. Eh, whatever floats your boat.
The reason why yogurt cups earned my curses was because the roots stuck to the non-slick inside of the cup (thus effectively killing more than half the roots) and because my yogurt cups weren't flexible enough to get all the dirt/root out in one piece. Which led to more root issues. I think soup/bean cans (if tin/metal) would most likely have the same problem.
If it's money that's tight, have you thought about using just plastic cups like Solo or something? You know, the ones that you always see in movies at parties with beer and such. Just poke a few holes in the bottom or on the sides just above the bottom, and viola! The cups are super flexible, inexpensive (less than $3/package I think?), *and* you can write the variety name on the side of the cup with a Sharpie to boot.