Didn't the suit Tom Cruise wore in Collateral have square quarters? I mean, it had a lot of stylistic quirks, but I'm just double-checking; my eyes might have deceived me. It looked pretty odd when I first noticed it, but then I couldn't imagine it having rounded quarters.
As for the OP's question, I would definitely go with rounded. I didn't think single-breasted peak lapel jackets usually had that many buttons, though.