What would one have to do differently if they used NaMnO4 instead of KMnO4 for redox titrations?
You are right. Preparing sodium permanganate is more expensive. The second reason is, sodium permanganate is hygroscopic and occurs as a monohydrate. This means that sodium permanganate is more soluble than potassium permanganate. (Solubility of $\ce{KMnO_4}$ is $0.065$ g/ml and that of $\ce{NaMnO_4}$ is $0.9$ g/ml)

So if you require a larger concentration of $\ce{MnO_4^-}$ in the solution, you can go for $\ce{NaMnO_4}$.