This is a good point to get hold of, and the converse of it may be as useful to think about: this is that going for single LEDs (plus series resistors) across the supply can lead to really low efficiencies. Since a very big current is then required at a low voltage, the voltage losses in things like rectifiers become quite large in proportion. That is not good news with any kind of supply, but with a linear one the result can be particularly dismal....If the LEDs are of the same current rating, you can actually more or less mix and match them in a given string; it frequently works out that a higher voltage (like 24v, 28v, 32v, etc) works out to be more efficient, as you can make the forward voltages all total up to be nearly the same...