Good Call Greg S. I think its important to realise that instead of investing the money into paying the property off, you should get another property, as the ROI would be better than earning a 4.75% (or whatever your interest rate on the refi is). Its basically sounds like your ROI is infinity (except for your sweat equity and time) so infinity beats 4.75% all day.
Of course at some point in the game, you are going to stop investing in new properties, so my advice is to sit down, analyse the NUMBER of properties you want to determine exactly how much net worth you want to accumulate.
My number is 25 properties. I'm currently at 5 and have a BRRR refi coming in to bump me to 8. I've calculated 25 properties will accumulate to be roughly $4 mil in equity when I cash them all out 20-25 yrs from now, and thats my number I am happy with. Determine your goal first, then make the moves to get you to that number of properties. Once you reach the number of properties you want, THEN start paying them off.