Your lender has a mortgage salesperson, who you are probably working with, who knows next to nothing about mortgages and is following a documents checklist. Then, there is the underwriter, who is the real expert, and probably won't look at your return until a week or so before you want to close. If the underwriter sees a red flag, they are going to ask questions and it could really upset the deal.
As mentioned, the underwriter will look at your adjusted gross income (line 11 on form 1040), more than your taxable income on line 15. I can't think of any reason not to take the allowed deduction, and I expect the underwriter will ask questions if you don't.
Note that claiming fewer deductions than you are entitled to can sometimes be considered bank fraud or tax fraud, depending on the circumstances. This applies mostly to self-employed individuals filing a schedule C (or landlords filing schedule E, I suppose). If you exaggerate your gross income by failing to claim legitimate expenses, that can be considered mortgage fraud. Good luck.
