Matt Whiting wrote:
Engines have vibration and resonances that vary with RPM. Running at a
constant RPM for long periods of time causes a certain wear pattern on
certain parts. Varying RPM over time induces different vibration an
part resonances and spreads the wear over different areas. This isn't a
bad, bad, bad thing.

My mechanic echoed this also. I was told even in cruise that it's a good
idea to vary the RPMs every 10 minutes or so.