Head of Household - Living Expenses?

I am moving in with my boyfriend and have one more year of my daughter in college that I can claim Head of Household.

The qualifications are that HOH has to pay more than 1/2 living expenses.  As my boyfriend and I decide who pays what, we both. were going to contribute a set portion to the joint account to pay the mortgage, utilities, groceries etc.  

Would it be better for me to pay those bills directly (or at least 50% of the them) and use the joint account instead as non-living expenses?

How would the IRS determine what all our living expenses are - mortgage, and utility bills are easy, but would they look at grocery bills?

Am I overthinking this?