Fix a flat is a wonderful product and does work as advertised and can get you off the road so that you can get on your way.
But if you can be sure to have a spare tire so that you can change the tire without the use of Fix a Flat because Fix a Flat can indeed do damage to a tire.
The Fix a Flat creates a mess inside the tire and rims and may require extra money to be charged by the tire repair shop because they may have to clean off your rim before mounting a new tire on the rim.
Sometimes the tire will be damaged as a result of the foam that hardens and then the tire needs replaced instead of patched.