It truly depends on what you want. A $100,000 property with 20% down and annual appreciation of 6% average, provides a ($6000 minus interest)/$20,000 rate of return. Assume an interest rate of 5.5% that would mean $6000-$4000=2,000/20,000=10%. If you have a renter and you break even on cash flow (including interest) then your profit would be $6000/$20,000 = 30%. This also assumes you don`t sell in the first year and have to pay lawyer and real estate costs.
On the other extreme, if you have no loan, and your profit would be: $6000 appreciation plus a tax free rental income of $4000 (i.e. the interest you would otherwise have to pay to the bank with 20% down). Your total profit would be $10,000/100,000 of 10%. I say tax free rental income because you can commonly offset the rental income with building depreciation. For me, that is a pretty good rate of return and low risk. Everything in between is kind of like a sliding scale.
Of course, you can also have down years. Suppose the market goes down 6%. In that case, your appreciation is -$6000 and your loss (with neutral cash flow) would be $6000/$20000= -30%. Wow, that is like a bear market in the stock market!! If such losses continued for a couple of years you are wiped out. Chances are that won`t happen, but you would need at least 1.5 - 2 years to make up for the losses.
In case of an all cash deal, your loss would be $4000-$6000= -$2000 or 2%. That can be easily recovered even by GIC standards in under a year. And remember, if you don`t sell you don`t incur the loss (right away) while having still $4000 income.
Now you have to look at your investment objective. What do you want to achieve: appreciation, or cash flow, or something in between (i.e. the sliding scale)?
For me, cash flow is important, but I don`t mind some appreciation as well. I have also learned, the hard way, how traumatic it can be if the housing market collapses and you can`t rent out the place or get a rent that is even less than the mortgage. So I seldom have 80% leverage - I feel more comfortable at 60% or in some cases no leverage at all (depending on how reliable the rental income stream of a property is).
It does not matter that you can sit out the bad times if your temporary losses are making you lose sleep at night. Peace of investment mind is very important - it stops you from panicking and from making emotional decisions. Also, it depends on what you do with your positive cash flow - do you need it to cover your cost of living or can you do without?
I think, for you to make a decision as to how much leverage you want to use requires the kind of analysis above. Use a spreadsheet for various `what-if` scenarios and see how the various scenario results would affect you. Then you can make a truly informed decision. As all ways, before you jump: "Investor know your self"
Hope this helps.