There are times when a copper fill can be used and times where it should not. Stitching is also to be used with precautions and is good where the integrity of voltage planes is not compromised by doing the stitching. It gets complicated.
If you have a simple circuit where all teh traces are on one layer and contained in a small area, a voltage referenced copper pour that encompasses that trace area and has multiple points of attachment to one of the voltage planes can help reduce the impedance of the power supply. Below is an example. It is a 4 layer board. Layer 1 (top) is a ground plane, layer 2 is a Vcc plane, layer 3 is another ground plane. The 4th layer (pictured) is the one with traces. The copper pour around the traces is connected to the Vcc plane at several (5) low impedance places. It's only purpose is to reduce power supply impedance and is not there to be a ground plane for the signal traces. If you wanted, it could be stitched to the Vcc plane if you didn't go overboard to the extent that the ground planes get compromised. But in this case, it's not really nesseccary.