I'm going to try using whipped white chocolate ganache as a filling for the first time and I'm wondering if I need to pipe a dam or not. TIA
No, i have never piped a dam when using Ganache - whether white chocolate or milk/dark chocolate.
As long as you have the consistancey right (thickish peanut butter ) you will be fine.
After filling your cake - allow it to *settle* so as any extra can escape before finising off your cake.