I bought a house that has existing Nandina growing along the back and one side. I was told by a friend that I should cut it back to ground level every year. Doesn't make sense to me, am I supposed to do this?

Nandina maintenence

There is no need to cut your Nandinas to the ground. If you wish to trim them, take out the offending shoots. Too tall? take out the taller stalks, cut them at least half the finished height. Too bushy? take out the low growth.

Nandinas should never be sheared, their beauty is in their structure and how the leaves fan out. Trimming them this way retains their natural character and even seemingly severe trimmings will quickly rebound. Since you don't disturb the tips of the remaining growth this way the recovery is instantaneous.