In general, the only reason to slope steam lines is for condensate drainage, as you suggest. In a boiler room or other operating facility, you need to consider the hot and cold shapes of the line, and slope the piping to the extent necessary to make sure that the line does not collect condensate in a pocket during operation and/or shutdown.

Traditionally, we assume that the way to do this is to make sure that horizontal runs of pipe adjacent to risers slope so that the difference in elevations at each end of the horizontal run is more than the vertical growth of the riser. Of course, we take the support system into account, but normally we arrange steam piping so that risers grow downward.

When drip legs are provided at elevation changes, there is no longer any need to slope the piping since the drip leg provides a collection point that can be trapped and drained in service.