Never, under any circumstances, use tap water for your humidifier unless the humidifier explicitly allows the use of it. Tap water is rife with bacteria, chlorine, fluoride, and other nasties. These agents can mold your cigars and humidor and negatively affect the flavor and consistency of your cigars. There are some humidifiers on the market that accept tap water and are able to contain contaminates, but I still suggest the use of propylene solution or distilled water in them if possible. In my opinion, 50/50 propylene solution is the best bet. This is a solution of 50% distilled water and 50% propylene glycol. The propylene glycol not only regulates humidity to 70%, it is also a mold inhibitor. Some smokers say they can taste the propylene glycol and others don't like the idea of adding chemicals to their cigar stash, but it is odorless and colorless, while being non toxic. Distilled water is void of anything besides H2O and is boiled and evaporated before bottling.
Distilled water is also a requirement on many humidifiers like the Diamond Crown humidifier, which has a wick that will clog up if PG solution is used. If you don't have any PG solution, distilled water will work just fine and is the cheapest and most readily available humidifying medium. However, since it does not have the mold inhibiting properties of PG solution, there is a bigger risk that you may encounter mold, but many cigar smokers use distilled water with no problems as long as they keep an eye on their cigars.