Best practice for multiple SMPS-generated power rails at different voltages on PCB
It all depends.
Let's just start by assuming that the ideal (or at least, most straightforward) way is to have a single converter for each output voltage straight from the raw voltage source. In that case, we run into beat frequencies if the converters do not have synchronized clocks and we have to deal with by using filters to stop high frequency noise from passing straight through the converters in either direction and contaminating each other.
But sometimes the voltage step is too large to be feasible in a single converter. In that case you might benefit form multiple cascaded converters to gradually step down the voltage. In this case again, you can have beat beat frequency and noise issues.
Or perhaps, the distance between the raw voltage source and the final load is large so you step it down a bit with an initial converter (and maybe use that to power other things as well), but keep the voltage relatively high to minimize currents and thus transmission losses and distribute it to converters closer at the load before stepping it down at the point-of-load (PoE).
Then there's other factors. For example, some regulator architectures are really good at stepping down the voltage by a large amount but are rather noisy, while others can only step the voltage down by a small amount but are quiet. You might choose how to arrange things based on where things make use of their strengths the best.