Welcome to Android Central! To be honest, I wouldn't worry too much about it. "Junk cleaner" apps might be more trouble than they're worth, since they often claim to be "memory booster" apps as well, and do things that are counterproductive to how Android is designed to work. Other kinds of junk cleaners might actually do things you don't want, like automatically uninstall apps you haven't used in a month or so (without warning you).
It's true that sometimes apps may leave behind some data directories after uninstallation. I periodically look through the directories using a file manager app and delete any directory that clearly belongs to an uninstalled app. But this often doesn't account for much storage anyway.
Clearing app caches can be done without a 3rd party app--just go to Settings>Storage, tap Cached Data, then agree to clear the caches. For the most part, though, there's no need to do this if the phone is otherwise working normally.
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