I do this all the time. I just put a few drops. Sometimes I'll add a little vinegar (like a tablespoonful or so) to help keep mold from growing on the insides, deodorize my home, and keep me from getting sick (not sure if this really helps, but I seem to think so
). I don't smell the vinegar at all. There was a time where everybody kept getting sick at work and I had this humidifier on at home with Lavender and sometimes a little eucalyptus or tea tree oil. I took plenty of vitamins and supplements, washed my hands a little more often than I regularly do (like sometimes I get lazy and don't wash my hands before I eat since I've been sitting at my own desk all day), and I didn't get the same throat viral infection everybody else did. If I even FELT something that could remotely be a sore throat, I gargled with hot water and tea tree oil.
It was miraculous.
Essential oils and vinegar really rock my world.