Is tap water ok for my seeds and plants?

Tap water in most areas is highly chlorinated, this does not appear to harm Cannabis; and countless fine crops are raised with water straight from the tap.

Chlorine is introduced to water as the gas Cl(subscript)2 which formulates slightly acidic water, the chlorine could possibly influence the plants indirectly, by killing some beneficial microorganisms in the soil; however, neither effect is expected to be serious. Organochlorine takes longer than chlorine to dissipate, so the best procedure is allowing the water to stand in an open container for three days at room temperature.

If the water has dissolved CO2 in it (like some bottled water labeled as pure), the pH will rise as that comes out. Letting it sit out is used to evaporate the chlorine, which is fine if thats what your district uses. Some areas have switched to chloramine, which is chlorine and ammonia. This will not evaporate and you need a reverse-osmosis filter (or a distiller) to remove chloramine.

To determine which is in your water, have a sample tested or call your utility company. Even with chlorine or chloramine, your plants may grow fine from the tap, but if you wouldn't drink it, why should your plants?