The bottom line is, how much do you want in your paycheck every two weeks vs how much do you want at the end of the year as a refund. The exemptions on your W-4 are a guideline for your employer to withhold money every pay period based on an IRS table for your income. These tables change every year. Some administrations rig them to give you more in your paycheck to make it seem like you are doing better, but you get hit at tax time.
When you claim the exemptions for yourself, your child and for HOH, you are getting these dollars now as opposed to later. You may be disappointed next year with your refund. On the other hand, if the money helps you more now, and you know that your refund might not be as big, claim it now. You can choose anywhere between zero (withholding the most) to three.