Great question! Because your second job is hourly and income varies you do not want nothing withheld, even though you are withholding extra in your salaried job, it still may not be enough. The hourly job is the smaller of your two incomes. Checking the box will ensure something is withheld. You can try not checking the box but make sure there is even a minimal withholding for federal. If not you can go back anytime and adjust W4. I hope this clarifiies. I don't want you to owe at the end of the year.
