I would be tempted to ignore a fire alarm if in low risk premises from which escape is easy.
In a higher risk situation I would evacuate promptly.

Low risk IMHO includes ground floor, small simple premises with plenty of exits and daylight.
Small shops, bars and restaurants (ground floor only) especialy if I am familiar with the premises.
The ground floor only of large retail outlets.
Ground level railway stations.

In higher risk premise I would probably evacuate promptly, examples of what I would consider to be high risk include theatres, cinemas and similar crowded places, anything not on the ground floor, and workplaces handling hazardous materials.