This year (2015) I left on my trip knowing that my tires were going to make it through the trip but would have to be replaced before my 2016 Travels.
I left in January with all 4 tires with the same tread wear, but an early morning drive to the dealer in Tucson for a 5000 mile servicing I cut into a turn lane and hit the front drivers tire in a sharp granite curb blowing one tire.
Side to side tire tres size is important for geared axles (only front of Prius) so I bought two tires that day and kept the two rear tires as when the tires get rotated the two front go to the back and the two rear go to the front. This way the same tread size was always side to side.
Now I'm less than 2 weeks away from my 2016 Travels so I ordered the new tires last week to go on the rear and had them put on this morning. ($200)
As I have read on the web, I find that I have slightly improved MPG running the tire pressure 1-2 psi higher than what is recommended. This also compensates for my travel weight too.