If you have only one kind of ambient light, (tungsten, fluorescent, or whatever) then gelling the flash is an easy way to avoid lots of post-processing work. But if you're shooting in a room with mixed fluorescent and tungsten and sunlight, manually blending multiple developments of a RAW with different WB settings is your only option, unless you can avoid the whole mess by converting to B&W. Each approach as its advantages and disadvantages. The multiproccess/manual blend technique will work in any combination of lighting, but requires a lot of post work. Flash gel requires minimal or no extra post work but is not usable in mixed ambient lighting. So you need to know both, and when to use (and charge for) each.