copper pour pcb
I use Cadence Allegro PCB Editor and have found that it is best to do copper pouring towards the end of the layout.
Ususally GND or critical power nets are poured.
If using Cadence, it is difficult to edit or create new routes over existing copper pour areas. Hence it is always better to plan which nets need to be poured, allocate areas during component placement and tackle copper pours towards the end of the layout.
But, certain critical pours like that of a heat sink under an IC can be done earlier itself, so that it acts as a reminder to you not to run routes or vias in that place.
Also, if it is a multi-layer PCB, you may assign the inner layers for GND or power. these can be copper poured and take care not to form too many copper pour islands in these inner layers. Else, the idea of a dedicated GND or power plane is compromised.