It depends...
Are you using a tent or a room?
If a tent, try placing the humidifer outside of the tent and letting the intake suck in the mist. However, humidifiers vary in intensity and efficiency. Also, most humidifiers run out of water after a few hours. I would recommend trying multiple scenarios and finding what works with your situation. The key is, monitor often. A humidifier can raise the RH realllly fast so be careful. Also, if you are using a HPS or MH, I'm pretty sure moisture can cause them to explode. So, be careful not to venture too far above 50%, which i believe condensation begins to form.
Hope that helps, but try short intervals of use at different locations and determine what gets the level where you want it.
Also, watering/drying etc will fluctuate the RH levels so it really is a constant game of adjustments. It is practically impossible to have an ideal set up that can go unmonitored, like a lighting system.