Depends on your risk tolerance - personally I want to maximize my leverage. I currently have 2 properties (+1 more under contract as of yesterday). My plan for property #4 is to finance it by taking out a HELOC then using that as a down payment on another multi-family property.
Here's how I look at it - for each $30k I spend I can get a 3 unit MFR in the markets I'm investing in (using 75 LTV). If after all expenses that gives me $800/mo in cashflow then I am getting a return north of 30%+/yr.
With $120k I could buy 4 properties, generating $3200/mo in cashflow.
If I did the same thing with all cash ($120k) maybe my cashflow for that property is now $1400/mo instead of $800. It's basically guaranteed to always cashflow but now I'm fully tied to just 1 property (not diversified), only have 1 property gaining appreciation, and my cashflow is less than half of what I'd get using leverage.
OPM is a wonderful thing, at these interest rates if you can find good deals I'd say leverage up to the level you are comfortable with/can qualify for.