If you wonder why you sometimes wake up with a raw throat and sore nose, think for a moment about how a dehydrator works. These devices create jerky from meat by passing a constant stream of warm, dry air over the surface. Rather than cooking the meat, the air slowly sucks the water out. Humidifiers can alleviate some of these discomforts, but follow the manufacturer's directions.
Purified water is the medium of choice where humidifiers are concerned, especially "warm mist" humidifiers that vaporize the water by boiling it. Using tap water can and will leave calcium and rust deposits on the heating elements, which will eventually coat them and result in failure. Aside from that, the chlorine used to treat city water can also aeresolize, and that's not something you want to breathe all the time.
If you've bought a humidifier secondhand or have a habit of running tap water through it, odds are it could use a good cleaning. Vinegar will dissolve calcium buildup if you let it sit for long enough and get it hot; simply fill the reservoir with pure vinegar, turn the machine on and set it outside to run overnight. Commercially-available coffee-pot cleaners will work just as well, and maybe a bit faster. Products like Lime-A-Way and CLR will clean the humidifier very quickly, but don't use them since you may have to breathe the chemicals later. Clean the humidifier every month or two to keep it working for many years to come.
You can use a water-soluble scented additive to add some atmosphere, but don't use it all the time. Bear in mind that scented "rose water" isn't really made from roses; it's a complex chemical compound that happens to smell like roses. While harmless when used in the prescribed manner, boiling and aeresolizing these compounds can break the molecules apart and may have unintended side effects.
Things Not to Use
Do not, under any circumstances, add any commercially-available "odor neutralizing" liquid to your reservoir tank. These products don't actually do anything to the odor in the room; they contain diluted ethanol (the same substance used in gasoline), which deadens the scent receptors in your nose. So the odor is still there, but you just can't smell it. Boiling and inhaling ethanol is just a bad idea all around, so don't do it. Fragrance oils might seem like a good choice, but oil doesn't mix with water. Even if the oils do manage to emulsify and make it to the heating coil, they'll more than likely cook onto it instead of contributing scent. Scented oils may work in "ultrasonic" humidifiers, but allowing minute particles of oil to constantly coat your lungs is asking for trouble.