Most devices will select the best channel on boot up only, and generally speaking they aren't good at it.
If you are using 2.4 GHz band, I recommend you to use channels 1, 6, 11 (wide bandwidth) to avoid interference, though they aren't the very best for all cases.
For the optimal results use this WiFi Analyzer app, it will tell you the best channel to use for both bands, even if nearby WiFis are not turned on all the time you can always open the app multiple times during the day, then write down the best channels, and select the common best one.
