Is it true that extra skimming over the joints is usually not necessary when using a glass fibre joint filler compound?
Generally speaking yes, glass and cellulose fibre reinforced plaster contains a high percentage of alpha-plaster, which does not shrink when dry, thus the material filled into the gaps and grouts fills the space completely, and it does not shrink when it gets hard.
On the other hand, extra skimming can be spared only if no drywall joint tape had been used. Extra skimming may be necessary if drywall joint tape is used.