Dust

Mechanical items like turntables and Tape decks should be covered. It is not only dust you are protecting them from but other pollution like cooking fumes. Purely electronic devices could care less. I just wipe them off with a damp cloth once in a while. For units with fans it is a good idea to blow them out with compressed air once in a while. 

Actually, heatsinks and other components that run hot are impacted by dust which is like a wool coat to a transistor.  Not a good thing.  I have always covered my gear with DIY black synthetic fleece covers.

I use a vacuum pump to remove all the air from the room whenever I am not actively listening. This stabilizes pressure on both sides of all vacuum tubes leading to massively increased tube life. Some of my tubes have now lasted longer than since the date of manufacture. Then all the air returns via Biosafety Level III filtration systems. BSL3 is the new audiophile gold standard. They will tell you it doesn’t matter but I can say for a fact the filters are directional.

I use dust covers on my tube and SS integrateds from radiodustcovers.com
Stan will make them to order and even embroider the manufacturer's logo, if you want.