Actually, IIRC, there are lots of imperial controlled worlds that don't get tithed for guard regiments, for a variety of reasons.
Some worlds simply have too small a population for it to be worthwhile. If I recall, Gravlax (From the first Cain book) falls into this category. It's almost certainly classified as a "Civilized World", with at least one rather sizable city, but it isn't tithed for guard regiments.
Other worlds don't give Guard regiments for political or economic reasons. AdMech Forge Worlds don't generate guard regiments, and Scintilla (From one of the Dark Heresy books), despite it's very large population, only recently created a guard regiment. In Scintilla's case, the planet is very rich in minerals, and that large population is used to toil away in the mines, gathering ores to be shipped off to forge elsewhere in the sector. They need miners more than they need guardsmen, so it's left alone.
I imagine that many Agri Worlds may also be exempt from Guard tithes, especially if they're growing the food that feeds an important hive world. Other worlds may simply not be called upon to raise a regiment until one is needed.