2.4GHz can set at 40MHz unless you have lots of smart switches.

5GHz can set to 80MHz just to adhere to the AC standard (wireless N is too old liao... so unless if you got devices from >10 years ago, otherwise AC should be safe).

5GHz is a bit of a mix bag because there are some router which uses higher channels (3 digits) to prevent interference from neighbour router but there are also those router which prefer to stick with lower channel (36-64) which provide compatibility with older devices.