More information here: https://investfourmore.com/2013/06/09/should-you-pay-off-a-mortgage-early-or-save-cash-flow/
I used the snowball method to pay off my first rental property in three years. Now I save my cash flow from my rentals in order to flip more houses and buy more rentals. If you hate debt maybe paying them off is better, if you want to buy as many properties as you can maybe you should save that cash flow.
Many people feel all debt is bad, but debt can be a great tool if used the right way. I use debt to buy more properties and make more money. I always buy below market value, with plenty of cash flow to make sure I am not risking too much.