So with an unregulated power supply, don't try to stack-up as many LEDs as possible in order to minimize the voltage across (and power wasted in) the resistor.
I would suggest that that advice be taken together with consideration for efficiency. You want the led string to have a combined forward drop as close to the main's voltage to maximize proficiency. However, that creates bigger current variability when the mains fluctuates.
You will need to compromise between the two.