I would suggest you keep your kitchen cabinet at 600mm rather than 620mm. I was told that contractors will do costing for cabinets based on multiples of 300 or 600mm. anything outside this "standard" multiples will cost slightly more due to more material wastage. this is because the standard plywood sheets comes in sizes of just slightly bigger than this 600mm multiple.
for example, the plywood sheet is 2400mm x 1220mm. so if your cabinet is 600mm, the carpenters can cut 4 pieces of 600mm x 1200mm out of the sheet. but if your width is 620mm, they can only cut 3 pieces of 620mm x 1200mm out of that sheet. of cos that last bit of about 550mm x 1220mm still can be used for other things but wastage is higher since the dimensions of left over is odd sized already.
hence it would be better to adjust your cabinet size rather than hack the wall to cater for a slightly deeper cabinet. After all, how much gains can you get from the extra 20mm of space?