No, you can adjust the flash output using FEC (Flash Exposure Compensation), adding an offset to the value calculated by the camera. On external flashes you can use manual flash settings where you define the output power is small increments.
For Canon you can read up here:
http://www.photonotes.org/articles/eos-flash/ Do take note of different metering methods for flash when using Av or M on Canon.
For other brands please check with manual and other online sources.