My left front tire of my 2005 Prius is wearing more on the left edge than my other tires. I have serviced this car with the dealer every 5K. They do rotate the tires and I did confirm the alignment is fine. What they did tell me was that I need to check my air pressure weekly and that I may be turning left more than right which would cause the wear on the left side of the one tire. What do you all think of that answer?
I would find someone competent to check your alignment. It’s importnt to have your tires correctly inflated, but it’s unlikely that improper inflation is causing wear on on edge of one tire.
Take the Prius to an alignment shop other than the dealer. Underinflated tires don’t wear on just one edge. Improper alignment makes a tire wear on one edge. The alignment might be “within spec” but not right on.
You need an alignment or parts replaced to enable proper alignment. Too low a pressure will cause wear on both edges, and to high pressure will cause wear on the center and not the edges, but wear on one edge is alignment beyond what these folks can figure out.
The answer is wrong, and this answer was probably given to you by a service advisor. Most SAs know little about cars and try to BS their way through any situation.
Excessive wear on the outer edge of the left tire points to either too much positive camber or too much toe-in.
During an alignment, caster, camber, and toe should be checked, and adjusted if possible, in that order as one affects the other.
The first thing I would look at would be camber since toe will usually affect both front tires to some extent.
Turning left too much is not correct unless you’re racing a lot and going the “wrong way”. 
Doesn’t NASCAR race with only left turns? Therefore, the only way the left side of the tires would be worn would be if the driver was going the ‘correct’ way. Personally, though, I think that real racers turn both directions.
It’s all left turns except for road courses.
The poster has wear on the OUTSIDE edge of the left front.
Turning left on the NASCAR track is going to apply pressure on the right front tire and the INSIDE edge of the left front. (although in theory they want the contact patch across the entire tire surface)
Going the wrong way is going to be the reverse.
If you are using the factory recommended tire pressures of 35psi front, 33psi rear on the Prius, you are likely to see outer edge wear (both sides) on the tires. The only way to combat this is to check the pressures regularly (so no seepage), or use higher tire pressures (but less than the max. cold pressure as listed on the tire’s sidewall. OE tires are the Goodyear Integrity, max. cold 44psi). I’ve never had my tires properly inflated by the dealership, especially following a rotation. (The tires were low when it was delivered! sometimes they don’t adjust the tires after rotations (so the rears improperly higher than the fronts), or they blindly put 32 (or less!) psi in all…) yes, some of the outer edge wear can be caused by lots of curvy roads, or turning your tires while parked/unmoving, but that should even out to wear on both sides…
If only on one edge, or seen on only one tire, it sounds like an alignment issue. Alignment is only covered for 12,000 miles/1 year on a new Prius, so you’ll have to pay for the alignment yourself on your 2005. (Definitely get the alignment done when putting on a new set of tires!) You could’ve hit a pothole or curb and that could have knocked that wheel out of alignment.