When clearing my androids cache should I go into my settings and do it manually myself or can I rely on an app to to clear it properly? Google playstore offers many phone cleaning, cache cleaning apps but I noticed that after I installed one app in particular, I went back into my storage just to check out the levels and they were not cleared. Could this just be a faulty app or are they pretty much useless?
Clearing cache is not something you need to do regularly. You do that when some app is misbehaving. 

As a thumb rule it's better to do things manually rather than using apps - ads, malware, unreliability are some of the reasons. 

If at all you want to install a phone clearing app make sure it is from a reliable source and doesn't infect your device. Google about the app in question !