It is ONLY okay to not use a dechlorinator if A) you let the water sit out overnight or at least 12 hours. This will dissipate the chorine. AND B) your water supply is NOT treated with chloramine.
Chloramine is absolutely lethal to fish and will not dissipate, no matter how long you let your water sit. Your municipal government/city hall/whatever you have where you are? should be able to tell you whether or not there's chloramine in the local water supply.