its ok to do something like that since it will shorten your typing later especially if you will use a variable many times.
the other reason using $POST (or any other superglobal) is because the default configuration of php for many versions now has been setup so register_globals (the setting that automatically puts $POST['name'] to $name) is switched off. therefore, as far as portability goes its best to do it. also there is some security involved as well.
take the following
if ($autorized == true) {
//sensitive data here
} else {
echo "go away";
}
that simply checks for a value of authorized being true, which could come from a session or checking in a database.
a person could simply call protected.php?authorized=1 and that snippet of code would show the sensitive data. with register_globals off however, $authorized and $_GET['authorized'] would be two completely different variables and that script would still say go away.
as long as you know what data is coming from get or post and you check it for invalid values or run it through functions to make it impossible to do sql injections or execute code, you are ok