You have to decide for yourself what metrics you want to use to measure acceptable investment return and to determine when you want to stop acquiring properties, and begin just reducing the debt on what you own with your eventual goal free and clear ownership. 

If you measure the totality of your investment, mean cash flow, debt reduction, appreciation, rent growth etc you will always make more money in the areas of high demand like DC, Boston, Seattle etc than you will make in a pure cash flow market like you will in the mid west.There is always the possibility of your rate going higher, but thats the trade off of a lower rate with the arm.

What metric do you use to measure the class of the neighborhood? 

Certainly abating the friction surfaces are a prudent measure to minimize future costs.  

If so, by what measure? 

Maybe some sort of a meter exists that can measure how much energy is being emitted to each zone. 

The one with the lowest cost per result is the channel to go with.Some investors go crazy with segmenting and split by customer type such as probate, foreclosure, divorce, etc.You can only manage what you measure! 

In the short term rents fluctuate, long term they rise due to inflation, scarcity of land, etc.One measure of property that is rarely talked about is what would this property look like as an investment if I had to lower the price to break even, would there be a demand for it? 

If so, how does this measure up to your investment standards?

Besides cash, are here any other precautionary measures you'd take to be in the best position possible?