I never had a cover for my ST in the old days. But i removed the srews for one or the other soldering and cleaned the board frequently.
The Apple IIsi, i bought last year was full of dust, i even couldn't insert a disk.
I clean my PC frequently to keep at least the fans in working condition.
TI 99/4A, C64, 800XL, 260ST, 520ST, 520ST+, 1040ST, 1040 STE, Mega ST1, Mega ST2, Mega STe, Apple IIsi