For transactions under about 30€ to 40€, your Chip & signature should work very well. This includes Navigo weekly fares as well as booklet purchases of tickets t+ (14.90€). These amounts are probably below the no CVM limits that most users have even in the USA.
Purchases of monthly Navigo passes at 75.20€ will likely require a PIN for those who have Chip & signature cards. I have found greater acceptance of my US bank credit cards at SNCF stations (regular train stations in Paris) when loading and paying for the monthly Navigo fares.