I guess it's personal preference. Are you having an issues currently? If so, it's better to figure them out rather than just changing fluid and trying again. If you're not having issues, what I would do (and this is just me!) is drop the pan and change the filter, if for no other reason than to inspect the pan and see what's in there, for curiousities sake. I've heard differing opinions on which does a more complete job of changing fluid, and after using several different machines, some do better, some do worse. It might not change ALL the fluid, but if it's not in real bad shape, I'd do a pan drop and filter change. You can even do both if you really want to get all the old fluid out too.