I have the humidifier in the lower level by the air intake which is the same area where the reptiles are. I haven't been able to check the upper floors to see if there's much of a difference but I do plan to get a hygrometer this week so I can adjust the humidifier if needed.
I did this when my ex and I were living with her mom before getting a new place. We noticed a slight difference and it probably would have been more effective if it weren't for the shoddy duct work throughout the house.