Republicans violated that principle as soon as they controlled Congress by mandating "dynamic scoring" for things like tax cuts. Whether or not one believes in dynamic scoring (it is controversial), the fact that Republicans mandated its use for circustances where they think it helps (e.g., tax cuts) is a shicking corruption of the truth spell woven into CBO.
Now, Republicans are planning to get rid of the current head of CBO and replace him with an economist with appropriate conservative credentials.
This is extremely bad from an institutional perspective, and is a splendid example of the genius of Republicans. They understand how this kind of change, which the rest of the world hardly notices, has HUGE impact. They are also aware that, when Democrats get control of both chambers again, the public memory on this is sufficiently short term that they can make a HUGE deal about it and make it look like it is the Ds that are 'politicizing' the CBO.
To recast the old slogan of "give a man a fish/teach a man to fish." Give me a majority and I can pass legislation for a term. Alter the institution and I can screw things up for the long haul.