Good reasons have been stated for varying from the box speed by Ansel Adams et al for black & white film. Those reasons do not necessarily apply well to color film. As Ansel Adams pointed out many times, he preferred black & white because it gave him more latitude to vary the use of the film and the print, while color was more limited since color tolerates a much narrower range of variation before the colors are blown out.
My experience has shown that the best results with color film, C-41 or slides, is to stick to box speed and vary the saturation by film choice pr polarizing filter.
My experience has shown that the best results with black & white film is to shoot box speed unless one needs to change speed to cover the subject brightness range, have the subject as middle gray, or create a certain effect all based on well thought out principles rather than what one had for breakfast that day.