There is a big difference between smart string nodes and dumb strings. Since you said nodes, I will assume that you are talking about smart strings and with them, the simple answer is that the nodes need to run at the voltage that they are designed for. Running them at a voltage over the rated voltage can toast the control chips in the nodes and running them at too low of a voltage can cause the nodes to become eratic, just shut down, or freeze. The reason for using the higher voltage nodes is to reduce the effects of voltage drop which does allow you to run longer strings without the need for power injection but as said before, if it is a 12V string, then it needs to run at 12V. Applying 24V to a 12V string would toast the entire string. Lowering the voltage should not have much effect on the brightness of a smart string node unless you really get the voltage low and then you have a better chance of the string screwing up.