Like most things audio-related, it depends.
1. Bass, midrange or tweet? Diff frequenices are affected differently by obstructions.
2. Size of openings in grill? Fine mesh or large "diamond shaped" holes?
3. If on a subwoofer, are the grills far enough away from the surround to keep the speaker's surround from hitting it on large-excursions?
All very relevant questions.