If you change at the bank, you have a receipt for your claims, not unless the money changer also give receipt which the company recognized.
Unless one carry large wads of money for transactions that can not see the light of day, we go Bangkok, the average we will be changing into would be THB23,000, how much loss can we incurr??
The person asking the question, must be changing more than that amount.
You change from the banks you leave a legal paperwork trail, the rate is official trading rates & is usually lower than the money chaner gives & you are guaranteed genuine currency notes.
Of course money changers give better rates, but there are risks, as mentioned, forged notes.
Airport rates or any International transit stations in anywhere in the world, have the worst rates, as any travellers will know. They are for desperate people, who need to change whatever currency notes they have to pay for taxes, extra bagage fees and so on.
In short, Banks offer the average rate & is the safest....the rest, you have to take risks.