The environment list is created at init, it's literally placed right behind the C argument list as an array -- AUXV if you want to go read the ABI Specification for it.
Therefore, anything you grab using getenv() can be considered to be static (Barring use of setenv), so the proper and correct thing to do is shove the things you need into a variable at init. Unless you yourself are editing it, but you should still use a variable because variables are typed and getenv is not (Thinking along the lines of storing port information, or whatever, where you need to parse it into a string to get it into the environment, and then need to parse it out of a string). For things like $HOME, those only ever change once, and you should really have a list of those that you check, because you will want to check XDG_HOME_DIR, and a few other areas. So you will want those in a list anyway, might as well do it at creation time when the data is fresh.
Anything you set with setenv() only alters the your environment state, and that will carry down to newly created children at creation time. So the only reason I can think of why anyone would do this would be to communicate data to child processes. Except there are so, so many better and non-stringly typed ways to do this, including global variables. Child processes inherit copies(?) of their parent's state, you can just use that, so there is literally, NO reason ever to do this.