Do I need to use resistors if I have a step down regulator?
If the LED array Vf is known at a rated current and your step down regulator provides that, the current will be stable.

Any rise in the voltage regulator above this operating point will result in a rise in current according to the power ratings of the LED array.

e.g. if  3x3 array is used and anticipated Vf is 3.0 "nominal" per LED each rated at 1W then you expect 9W load at 9.0V.  But the tolerance on Vf depends totally on the ESR (&amp; Pd max ) of the diode.  This ESR is the incremental series resistance near rated power and typically now ranges from 1 Ω-watt (+/-50%) thus a 10W diode is 1/10Ω and a 1W diode is 1 Ω.


Then a 3x3 array or 3S3P array= the same ESR as 1 resistor or 1 Ω  in the case of 1W LEDs.


Thus you can expect current to rise 100mA for a 100mV increase of 9V
Considering a 1W 3V LED normally uses 330 mA 100mA rise imposes tighter regulation and possibly adjustable voltage, 
so a safer approach is to add the same value as the equivalent ESR or 1 Ω and raise the voltage by 1V so that the sensitivity to voltage change is now cut in half to 50mA using a 10V supply. 
Constant Current regulators using a 50mV current shunt are the ideal approach while an andjustable LDO with a 2.5V drop and a current shunt R to maintain constant I is less than ideal but stable and perhaps more suitable for 50V strings in a simple solution.



Note this array method assumes all LEDs are from the same good supplier and similar batch Vf ESR values to prevent thermal runaway at max Pd, which is more often true, but not always (ok?)