I do think it's worth mentioning that while the High Lords probably have zero interest and time to deal with individual planet's fertility policies, the nature of the planetary tithe of men for the Guard would encourage Planetary Governors to come up with policies to encourage a high birth rate so that the tithe could be meet.
Well, supposing that planet's tithe is in the form of fighting men. One would assume that mining worlds and agriculture/aquaculture worlds, and similarly highly specialized worlds could escape the demand to send 5% of their PDF to the Imperial Guard and instead sending their respective resources - much like Catachan
only provides fighting men, and not anything else (if my memory serves me right.) It's a nitpicky point, of course - I agree completely that an Imperial Governor would probably be concerned with how to sustain the population of his planet.
And to see how effective the Imperium is at repopulating planets - didn't they essentially replace the entire population of Armageddon in about a 400 years time?