Yes, I would've kept the PG950. Any cassette with at least one thin cog can dig into an alloy cassette body, which includes the PG970, so you wouldn't have accomplished anything. The 950 actually has all its thin cogs bolted together, too (1.5mm hex key from the backside) which helps transfer the load from any single cog to its neighbors.

Anyway, moot point with a Tiagra hub. As the others noted, it's got a steel freehub body.