In general, train the way you plan to ride/race. If you get stuck in lower cadences (from gearing or other reasons) and feel hindered at them, you should probably consider adding similar work to your training.
Make the transitions in small increments and short intervals at first. Gradually decrease the cadence and increase the interval duration (likely not at the same time) so you get down to the area you expect to need.
It may not be necessary in all cases, but broadening you cadence range is usually a good thing to do.