How are "yes" and "no" formatted in sentences?

Solution 1:

Unless there's a style guide telling you otherwise, I'd suggest basing your decision on whether you mean the literal words 'yes' and 'no', or the general nature of the response. Consider:

Why would he say, "No"? (For that is the word that he said.)

vs

Why would he say no? (What he actually said was "Over my dead body", but let's not worry about that detail.)

Solution 2:

In both the answer is no and say no, quotes are relatively uncommon. The Corpus of Contemporary American English gives these results:

the answer is no        484 hits
the answer is " no       27 hits (including punctuation variants)

[say] no               8891 hits
[say] " no "            521 hits (including punctuation variants)

However there are only 10 yesses and 30 yeses, so you might want to reword that one (perhaps The votes are in: 3 in favor and 4 against).

My subjective impression is that it is better style to omit the quotes. Your style, of course, is up to you.