You should use pulses not less than 10 milliseconds (ms) in length.
The autoguider port is polled approximately once per ms by the mount electronics. (The exact rate depends on many factors.) Since the accuracy of a pulse duration is 1 ms, sending a pulse whose duration is less than 10ms could result in errors larger than 10%. Sending a longer pulses could result in smaller relative errors but the absolute error is the same.
This pulse width requirement does not change with your guide rate setting because the timing of the autoguiding port polling does not change. The size of the error relative to a guiding displacement is independent of the guide rate. However, the absolute displacement error of a guiding pulse is proportional to rate. For example with a guide rate of 50% of sidereal, the absolute displacement errors would typically be +-7.5 milliarcseconds and with 100% guide rate, 15 milliacrseconds.