Like most footwear, Chaco sandals do require a break-in period. But unlike most footwear, you're going to be able to break them in quickly.
Chacos are going to be uncomfortable the first few days you wear them until they fully break in and conform to your foot's contours.
If blisters start to form on the bottom of your feet, it just means you're going to have to be more patient as this often indicates your sandals need more days to fully break-in.
To prevent blisters, try to alternate your Chacos with some really comfortable running shoes to give your feet some kind of recovery time before your Chacos become really comfortable.