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My house is old, and much of it has the original knob-and-tube. A prior owner rewired the 'wet' areas (kitchen, bathrooms) with new, grounded, wiring.

Adding GFCi protection to the remaining two-wire circuits would make them safer. Aside from 'things with motors', is there any reason to prefer going plug-by-plug to adding GCFi breakers?

Almost everything we plug in has a two-wire cord. But there are a few exceptions (air cleaners, a microwave that has wandered out of the kitchen, and a laser-printer which has a prior track-record of tripping GFCis, so it needs a proper grounded circuit). What are the considerations here? A GFCi outlet provides a three-wire connection (and a 'no equipment ground' sticker), but is plugging a three-wire plug into it dangerous?

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For most purposes that are reasonable to expect, there's no functional difference between a GFCI breaker and a GFCI outlet at the first position on the circuit. The odds of having a ground fault interaction with the wiring from the breaker box to the first outlet are generally quite low as it's inside walls, etc.

You don't need GFCI outlets everywhere unless you have a hard time remembering where the first outlet on a circuit is to reset the GFCI when needed. And you quickly lose the cost advantage (which favors "first outlet only" over breaker) when you have 2 or more GFCI outlets on the circuit. At that point a GFCI breaker is less costly, usually.

It is not dangerous to plug a three-wire cord into a GFCI-protected ungrounded outlet. That's why that configuration is blessed (with the no equipment ground stickers, as you noted.) Current flowing on the ground would have to come from the live conductors, and would trip the GFCI if it was more than 5 mA.

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