@Steve A. When determining the right amount of leverage, what it really comes down to is your own risk tolerance, investment style and goals. Personally, I have no fear of taking as many 80% 30-year fixed loans under 5% interest as I can possibly get. In his interview with CNBC, Warren Buffet said that he would love to buy a couple hundred thousand single family homes with 30-year mortgages at today's rates. The reason I say that is because it's obvious that this guy has all the cash in the world, but he's still choosing to leverage. Apart from the straight cash-on-cash ROI, you also need to account for the massive benefits of interest deductions. I get several thousand dollars a year back on my taxes due to the interest expense deduction - and that's just on my primary residence. I wouldn't have this if I bought cash. I also wouldn't have cash in the bank either.
Also, keep in mind the benefits of inflation. Over the decades, the dollar loses purchasing power. Example: what you could buy for one dollar back in 1984 would cost you $2.27 in 2014. So while your real estate may fluctuate in value in the short term, it is very likely to appreciate over a 30-year period. At the same time, your mortgage payments remain the same. And the loan balance continues to go down.
If you don't feel comfortable with leverage, that is perfectly fine. But in the title of this post you mentioned wanting to build a real estate empire. You definitely won't be able to do that with a $12k a month job and doing cash transactions on all of your investments. Don't get me wrong. You may do very well, but you won't build an empire without the appropriate leverage.