Stem length is all about fit. (At least for properly sized road bikes.) Yes, changing the length affects steering some but very rarely to anyplace unsafe.
I raced an 11 cm. Rode decades commuting on a 18 cm. I have long arms so I steer toward stems most would call very long. For me, stem length and
wheight ((edit: oops!) are critical elements in making a bike that works well for me. (And I figured out years ago that I can trade height for length and keep the position I love.) But as far as fork offset goes? Who cares? If you stay ignorant, that will never, ever matter.
Just my views after a few miles of riding a few different bikes.
And edit: The great Eddy Merckx (and by default, also God) was said to ride 12 cm stems. So by the rule of stem length equalling fork offset, I should have had all three of my custom frames built with 120mm fork rake. I'm guessing that a head tube angle of around 45 degrees would be needed to get proper steering. That would push my bikes' front centers forward about 15". To keep proper weight center over the wheels, I'd have to push the rear wheel back about 10". New wheelbase? ~65". New total bike length? About 92" or nearly 8'. Time to modify my garage for bike storage.
