I suspect taking out helocs or refinancing your existing properties is a better strategy than using a blanket mortgage, because you have more flexibility as Chris mentions.
I'm considering the same problem myself. I have equity in my properties, which I could borrow against "pyramid" into more properties. That's great as long as things keep working, but another big downturn might give that strategy a negative outcome.
I haven't figured out exactly what rules I'm going to work by on that, preferring for now to take the more conservative approach of buying only when I can use hard cash for the downpayments. I have thought about the problem, however, and I've got some suggestions.
1) Use re-advancable helocs for the downpayment funds, and never go more than 80% LTV. For the 'V' in that, don't use the highest appraised value you can get your hands on, instead, take the lower of the appraised value minus 10% and your purchase price plus 3% per year for inflation. That way you don't get caught borrowing against inflated property during a boom to purchase more inflated property during a boom.
2) Tighten your criteria for buying. If you currently require $X cashflow per property, require $2X cashflow when "pyramiding" with a borrowed downpayment.
3) Stress test your portfolio, including the debt used for downpayments. If it doesn't work with lower rents and higher rates, don't buy it with borrowed money.
Now, these are just some things you might want to think about, not my personal rules, as I haven't decided on anything as of yet.
I'd also be extremely interested in hearing the controls others use when deciding whether to increase their leverage.
Regards,
Michael