The only 'harm' might be to an SSD. All this needless cleaning and deleting is just introducing extra, unnecessary, write cycles to the drive(s).
And, you're actually making your system slower.
Temp files exist so that the system does not have to go out to the web to retrieve something. It can get it from internal storage, that Temp folder, much much faster.
I'm not saying "never".so i should leave everything and never delete it again?
I'm not saying "never".
Just not obsessively, all the time.
Windows does a pretty good job of cleaning itself up after a while.
And by "do system maintenance "...what do you mean, exactly?
Control Panel>Security and maintenance>Maintenance tab.There is an option called ''Automatic maintenance'' which i do manually every day.