If you refi'd into a 30 year loan, you can still pay the extra monthly principal payments to pay it off in 20 years, but you always have the choice to pay the less amount and possibly cashflow, if you wanted.

Just to make sure I understand, if the property is producing positive cash flow to at least cover the mortgage principal, interest and taxes, that cash flow negates the debt of that mortgage which keeps my DTI in line.

Is it like a traditional mortgage where the interest in the first year is high compared to the principal?

You can wait for appreciation and principal paydown after time and pull the money back out.

If you take a cheaper $80,000 property that rents for $1,000 a month, and you do a 75% mortgage on it, between your principal, interest, taxes, insurance, and maintenance reserve you will likely spend about $600-$700/month.  

I am applying the cash flow to principal pay down on the property, so we are gaining equity. 

I can sell and walk away with more $$, but then I have to redeploy that money, lose cash-flow and principal pay-down from a tenant.I decided to take out a HELOC ~85% LTV, keep the home (it will still cash flow) and use the HELOC proceeds to purchase further investment opportunities.

To make a 1% minimum monthly payment, you are paying another $343 in principal, and it will take 20+ years to pay it off (I haven't done the math, but your statements will show you this.). 

What are your thoughts on leveraging the first few on 15 year notes, then using your W2 income as well as the rental income to pay down the principal faster, then buying them in cash once you have a few properties paid for? 

The return wouldn't be exceptional but it would be paying down the principal with a little cashflow.