It is not a good idea to use tap water for your venus fly trap (Dionea) as tap water contains dissolved solids that can harm the plant such as sodium, calcium, chlorine, magnesium etc etc.
Over time these minerals will build up and eventually harm or kill your plant. It is highly recommended that all carnivorous plants should only be watered with distilled or better still demineralised water as this type of water contains little or none of these dissolved solids.
This also means that you would need to re-pot your plant less often as there will be less of a build-up of these minerals in the soil.
It is recommended that your venus fly trap is kept moist as opposed to over watering and the best way to do this is to spray your venus fly trap with distilled or demineralised water.