If you can fill it more without giving them an opportunity/way to escape, then do so. You will have to be the judge as you know your set-up, but giving your RES the most water to swim in that you can is never a bad thing. The only thing to watch for would be any possible escape routes.
We had a close call with Koopa this week, but my daughter just so happened to look over to see him stretching as far towards the top of the tank as he could. After adjusting his escape route where it couldn't happen again, I don't think he was too happy with me but I feel better.
