240V 5A PCB traces - stitch double sided or not?
Stitching (with vias I assume) will increase your cost. According to PCB trace calculator, for a 1oz copper and 5A load you need a trace about 3mm wide (for 10C warming over ambient). Can you afford this? If not, the standard trick is to beef up the trace with solder build-up, as manufactures do in millions of PC power supplies.

Stitching, however, will slightly improve thermal management of the trace, since vias will increase the heat exchange between two traces in cases of bad board orientation relative to gravity direction, when one side has a depressed air convection relative to the other side.