when installing a new SP farm, is it recommended to install the CU before or after running the products configuration wizard for the first time ?
Each time you install a Public Update, you must run the Config Wizard. The order you're asking about does not matter. You can either install RTM and then the PU you need to, then run the Config Wizard, or install RTM, run the Config Wizard, install the PU, run the Config Wizard.
Just don't forget that you must install both the sts-x-none and wssmui patches each month (with an exception of the Feb 2018 patch - only sts-x-none was released and you would use the wssmui patch from Jan 2018).