There is some evidence that diffraction can be audible, but no one really knows how audible these quantities are. The paper cited in the article that demonstrated the audibility of diffraction effects can be read in full
here. In a normal listening environment, diffraction caused by grille frames isn't likely to be a serious detractor from the sound quality- unless that grille was just terribly designed. As was discussed in the article, poor speaker placement is going to cause much worse diffraction than most grilles.