A hammer works to flatten meat and poultry, so it makes some sense that it would work on cotton. As long as the strength of the seams aren't threatened, I'd go ahead.
Geri B makes a good point about sewing from the center (of such a crowded intersection) out, but patterns don't always specify that. They should, though!
FWIW, I have been pressing almost all my (machine-sown) seams open for the last two years. While it can be a bother, I've encountered no problems in terms of seam strength and have begun to recommend it to almost everyone. It definitely distributes the seam fabric more evenly.