Definitely clean the humidifier often.  I would recommend every other day.  If you don’t, you will be releasing mold spores into the air which will cause more problem.  The other thing is that warm, dry air is the WORST thing for anyone with sinus problems.  That’s why the cool mist humidifier works so well.  My family has adjusted to keeping the house a little cooler in the winter than we used to (68 degrees instead of 70-72) and, despite my current sinus irritation, we all stay healthier.  I think right now I am fighting the change in temps from warm days to cold and rain/sleet today.  Good luck.