Using boiled tap water is NOT a good idea. What you are trying to avoid is the dissolved solids in the water. By boiling the water, you will actually increase the amount of dissolved solids because some of the pure water will evaporate and the dissolved solids will remain. So boiled water will actually have a higher amount of total dissolved solids than before it was boiled!
You can get a TDS (total dissolved solids) meter and measure your tap water to see if it has a TDS measurement of less than 100ppm. Most water from the tap does not. In most cases, the best option is to purchase bottled distilled water at the local grocery store.
You can get more information about watering your Venus flytraps on the watering Venus flytraps page.