Best way to save postmeta

Your approach is good, you should run a basic check against the data tough. Otherwise you might get an array or a serialized object … which might lead to unexpected consequences (including security issues!) when you or someone else is trying to print the values.

Maybe something like this is easier:

$key   = '_my_meta_key';
$value = filter_input( INPUT_POST, $key, FILTER_SANITIZE_STRING );

if ( $value )
    update_post_meta( $post_id, $key, $value );
else
    delete_post_meta( $post_id, $key );

Note that you should avoid writing the key string more than once. If you ever want to change its name, you should have as few places to search for it as possible. Ideally, just one in your whole code base.