TBH I always use the blue stuff that says -10°C to -1°C (15-30°F) in every weather. I think that waxing often is a lot more important than choosing the temperature-appropriate wax. I used to use warm weather wax in the spring and cold in the winter, but never noticed any difference. Now I use medium range wax for everything. I wax my skis after five or so days of skiing, and my skis always go way faster than any of my friends'.
So if you never wax your skis, just wax them with something. You will notice a huge difference. If you're already waxing regularly, then sure, start using weather-appropriate wax. You might notice a small difference. I didn't.