If you are filing married separate, then if your spouse files using itemized deductions, then you are required to file itemized, no matter which method is "better". TurboTax gives you the option on choosing standard or itemized. I think (relying on memory) that you search for "Itemized Deductions" and jump to the result; if this works, it will give you a screen that allows you to choose itemized even if TurboTax is suggesting standard. (Sorry, the TurboTax Help screens are coming up blank on previous answers, so I can't give you a useful link).
I don't know of any reason you can't choose itemized, even if there is no benefit to do so.
