When will parallel LED voltage drops become an issue
It's not an easy calculation. You can get some idea of the worst-case situation by looking at max/min forward voltage drops, and the slope of the current/voltage curve, and the LED temperature coefficient from the LED data sheet. 

Without detailed statistics on LED voltage-current-temperature characteristics it will be difficult to guess what will typically happen. Cheap consumer products often parallel LEDs directly with no resistors, but then nobody much cares if one LED is 30% brighter than the next and the whole thing only lasts a few thousand hours or less. 



Edit: 

Here is a simulation you can play with. I've altered the saturation current of the model for D2 to make the Vf high by 4%

40mA current splits with D1 getting 23.1mA and D2 getting 16.9mA. That's if they are all held at exactly the same temperature. If instead I assume that they are thermally independent and have a 50°C rise nominally, then the difference between the two would be 16°C and if the tempco is -1.7mV/K then that would cause another 1.7% difference between the Vf's, leading to more temperature rise etc. 







simulate this circuit &ndash; Schematic created using CircuitLab