What you can and can't do with the engine has to be backed up by data from the engine or airframe manufacturer. Many turbocharged engines require that full redline RPM be set when using the maximum available MP.
The allowable relationship between MP and RPM is model specific and will vary even for the same basic engine.
So to answer the OP's question I would ask your instructor to show you where the 38 in 2600 RPM power setting is authorized. Some POH's have a chart of allowable MP vs RPM which will provide the necessary data, however for most engines you have to go to the engine operation manual provided by the engine manufacturer.
Sadly the practice described seems to be the all to common flying school practice of procedures mindlessly passed down from instructor to instructor with no critical thought and unsupported by any actual data.