We use Christina's trick quite a bit. You can even make the answer to any "hidden" fields mandatory; they will only be mandatory when displaying to the user. For example:
QUESTION 1 choices:
• Blue
• Green
Question 1A is currently hidden, but is also mandatory, but only if displayed when Question 1's answer is "Blue." If the answer is "Green" then Question 1A will not display, and since it isn't showing, it being marked mandatory is ignored by WF.
Very handy.