I check for dust and clean the filters on my case every 1-2 months. But my workspace is really dusty.
It certainly won't hurt you to clean your system every so often and if you area is like mine, then 'yes' I would recommend doing so.
Fans can be run at 100% like you asked, or they can be throttled by your bios or a fan controller. MU is right that you should be fine letting your bios control your fan speeds.
But, again, if you want cooler temps then you leave them on 100%. Doing this though will most likely allow dust to build up faster, use a little more electricity and be much noisier than just letting the bios or a fan controller manage the fans.
Basically, none of the things that you asked about will cause any trouble for you either way. So it is really all about what you prefer