How do I decide what is "too bent" when I join a tongue and groove, and there ends up to be a little space in the middle, but the ends are touching?

Do some people "push" such pieces in to bend them back and while holding it down nail it in place, or should I be rejecting them?.. maybe cut in half and re-groove?
It's very hard to bend the groove side.. it would need a lot of pressure to close that gap.
Somebody told me "don't push it tight" while nailing it down. Is this correct?

Typically I think there shouldn't be room to fit a nail between the planks though?

You can always cut the board in half to use as a starter or ending on a wall. The longer the board the more leverage you will have prying it into place. A long screwdriver will usually coax anything that can make it through the moulder into place. A staple gun and a firm tap of the mallet will draw the last little bit into place. If the wood is still a little bit too dry from the mill, you can leave a few small gaps that will fill in as it finishes acclimation after it has been laid, but before sanding. You will need a moisture meter and some experience to know how tight or slack you have for the job.