Pay down mortgage or use interest deduction to limit taxes owed?

[Russell Buxton]

I would love advice on this investment question. I currently own 4 properties that I pay approx. 5k in total in mortgage interest payments per month. I could easily pay off all my mortgages and own all the properties free and clear. I do not know how to calculate whether it is better to maintain the mortgages and use the mortgage interest payments to offset any taxes on my profits (all properties cash flow well) e.g. I am worried that I will start to own taxes on my net returns if I do not have any mortgage interest deductions. How can I calculate at one point it is better to own all my properties free and clear -v- maximize interest deductions for tax purposes?  

I am aware that there is an opportunity cost that I could miss out on if I were to use my excess funds to pay off the mortgages in terms of using those funds to buy other rentals, but at this stage, my portfolio (8 rentals in total) is enough for me to handle.

Thanks