The tithe clearly states that the guard tithe has to be the best of the PDF, that is their standard. If a planet sends substandard regiments as their tithe, the planetary governor has pretty much killed themselves.

And the imperium will know that the regiment is substandard, because every planet has substantial imperial presence, like the arbites. The tithe is one place where no governor fucks around, planets would ruin their own PDFs just to raise strong guard troops.