As you may have seen from the slide show I placed in the forum I only use nets and that is only to keep Birds & cats off basically.
I think fleece might create problems rather than prevent them i.e. you are creating an atmosphere more conducive to attracting insect life rather than preventing it, then there might be fungicidal problems as you create a more humid atmosphere.
Keeping plants at recommended spacings and weeds at bay are probably a better option!