In our arid region where percentages drop below 20% in the winter and with the furnace running a lot...you bet we had to run a small cool mist humidifier 24/7.
We used a humidistat to monitor our numbers and in the most extreme conditions with furnace running a lot it was difficult to hold at 25%.
In some regions of the country it isn't necessary but believe me out west...it is vital!
The downside is the probability of freezing condensation on glass, window frames, walls and even inside cabinets, which will occur even at less than 20% so folks might as well be comfortable. We kept one door to each cabinet open all the time to equalize temp and humidity. Also, be sure cabinets aren't packed full and all items are pulled away from the wall. We had books and clothing get frozen to the wall!
Choose a size that fits your space. We had ours sitting on the kitchen counter so didn't want a big unit. It held enough water to get through the night. At home we find the 1 gallon model is just about right. Since they need to be cleaned out often the larger they are the more work to keep clean and more to find storage for off-season. It's no bother to refill ours 2 to 3 times per day.
2002 Tiffin Phaeton
2005 Malibu Maxx toad