If everything happens strictly synchronous to your audio clock, then fixed block processing is the way to go. But jblow is right in that when you have to feed

If everything happens strictly synchronous to your audio clock, then fixed block processing is the way to go. But jblow is right in that when you have to feed in samples from a non-synchronized source into your processing/game/video-application/... then trying to work with the fixed audio block size will be terrible/require additional synchronization somewhere else, such as a adaptive resampler on the input/output of your "main loop".