at the highest speed possible. well, sometimes the action in my scene may not be changing very quickly. this is particular true shooting portraits. continuous mode is a great tool for shooting portraits. because sometimes there can be very subtle changes in facial expression and they can be hard to predict, which makes it difficult to get them when shooting in single shot mode. but if i change to a continuous mode, i stand a better chance of getting just a subtle change that i like. facial expression may not be changing that quickly though.
so sometimes switching down to the slower continuous low mode is a better way to go. 