Hi Joel,
If you're using an Inport block to load the data, there's an "Interpolate data" option that will give you a staircase signal instead of interpolating.
However, why should Simulink's interpolation affect your results? If you are using precipitation data, shouldn't the amount of water falling be accumulated over time with some sort of Integrator block? In that case, it shouldn't really matter whether you sample your signal "too often".
Another thing you can do if you want to avoid additional time steps is to explicitly set the sample time of the input signal to match that of your data. However, I wouldn't necessarily recommend this since you may then have to make the sample times sync up with the rest of your model (depending on what blocks you have).
