Just started using mysqli. If I'm working with small data sets on small websites (traffic-wise), do I really need to use these all the time?
$result->close();
$mysqli->close();
Also, for someone doing custom PHP and MySQL work without a framework, is mysqli the general preferred way of interacting with MySQL?
PHP will close all open files and DB connections at the end of the script. It's good practice to do it manually when you are done with the connections, but it's no disaster if you don't. If you have a DB connection that will be used throughout the whole script you can as well leave it open.
+1 on PDO