I don't have much experience with designing boards for mains voltage. I've designed a circuit and I have lots of unused space on the board. Do I fill it with copper or leave it blank? I guess the PCB manufacturing house won't be very happy with all the copper to etch. If I copper-pour it, what should I connect it to? Earth?
First, thicken up the power traces to make absolutely sure that (if life gets interesting) fuses will blow or breakers will pop rather than your PCB traces vapourising. In my opinion a 0.25 inch track width is not too much.
Second, space out the traces to get at least 0.25" between live and neutral, and a bit more (0.5" if you can) between either and any other circuitry. This is an ideal; if a component forces less clearance, do what you have to do.
And I would prefer a bit more clearance rather than copper pour. But if the board is huge, I would consider a copper pour in areas more than 0.5 or 1 inch from mains circuitry.
There is no justification for such large clearances in the PCB materials themselves; I have seen guidelines that say 5mm is adequate. But as boards age, there may be contaminants... I have seen one board that would have survived longer if its live to signal clearances had been bigger than a large (carbonised) ant!
Have them thieve the layers. Lots of copper to remove means lots of etching. Lots of etching runs the risk of getting open. Thieving means they add lots of floating copper where there aren't any signals or planes. Your PCB software can do that or the PCB manufacturer can.