i think you mean the option to use the bank conversion rate vs the other 'local rate. forgot the exact wording... i never had time to calculate as i am afraid the atm will gobble my card. i have tried to google an answer but nothing clear/'concise was found. so, as you say... i always choose no but i wonder if that is the right choice.
i tried to make sennse of the kasikorn website for currency exchange cos i believe their rate is the best. but their heading that has buy/sell is somewhat confusing.
you might have misunderstood my op about krungsri vs bkk bnk. i ALMOST never use bkk bnk atm. they are/were limited to 25k withdrawal whereas krungsri is 30k. about 2 years ago i switched to kasikorn as they seem to have a better, although insignificant change, in their exchange rate.
i do find bkk bank for service is better than krungsri... but as you pointed out not xchange rate.