The max amount credit prize will be higher if you retrigger and it is more then your inital free spins. And when I do, I check to see what the credit prize is before I spin on. But I very rarely take the credit prize, during the free spin round.
You can accept the credit prize at any time within the free spins, as long as you have 20 free spins left. After the 20th free spin the credit prize option button goes away.