"All four tires have inside shoulder wear"

Tire wear on the inside is usually a function of too much negative camber. The rear camber is not adjustable on the Prius and I don't think the front is either. Was the car heavily loaded with luggage and 4 people (this can cause negative camber in an otherwise correctly aligned car)?

Critical problem? Well only to your wallet when you need new tires prematurely!

 

i don't understand why tire alignment is such an issue on the prius. i've never encountered this before on the 20 other cars i have owned. is it more widespread than i realize?

 

Inside shoulder wear means that the tire tread closest to the center of the car is wearing more than the rest of the tread. If you invest in a tire tread depth gauge you can measure the tread thickness across the tread and confirm or deny this finding.

The criticality of the problem depends upon whether the tread still has adequate depth on the inside shoulder or not. If the tread is bald on the inside shoulder and you plan to encounter rainy weather you may want to replace the tires before your next road trip. If you see plenty of tread (which to me means at least 4/32" tread depth in rainy weather) then no worries.

Front camber actually is adjustable. The procedure is to loosen the nuts on the two large bolts that hold the bottom of the front strut to the axle hub carrier, then move the strut in or out as needed to achieve negative or positive camber. If additional adjustment is needed, there are special bolts that can be purchased from the dealer to give you more range of motion of the strut.

The Prius suspension is close to "agricultural" in its design and won't win a prize for being world-class. Also the tires (at least on Classic and 2G non-Touring) are the minimum size needed to support the weight on the front axle. This doesn't help to promote even tire tread wear, but it's just a quirk that I live with.

 

If the OP has Goodyear Integrity tires (most of us did), they have very soft sidewalls and are prone to scrubbing the tire edges in turns. A good replacement tire will wear more evenly.

JeffD