If they are feeling "spongy" or "soft" you may want to take your car in to have the brakes looked at. Same if the brake pedal seems to go "too far" to the floor.
If something doesn't seem quite right with them, I take the car in to have them looked at. There's a lifetime warranty on mine, and it costs nothing to have them looked at.
Normally when i send my car to the service station they check the break power and i don't have to worry about the problem with breaks because of the frequent checkup.