The best bet, is to set the sockets so that they are in the centre of four tiles. This way you can easily cut out four corners, and get a neat balanced finish.
Probably the best way to proceed is to leave the boxes where they are for the moment and to measure and design the layout so it looks balanced and right.
Mark the patten on the wall and start tiling. Once you are sure the junction of the four tiles is set, cut the hole. Make sure the centre of the box lines up with the centre of the four tiles. Cut the tiles to overlap the box edge and use long screws to fit the socket.
By the way, keep in mind the current regulations regarding the location of sockets with regard to kitchen sinks - this may well require some changes in existing layouts.
Hi, doing ANY electrical work in your kitchen is illegal unless you employ a registered electrician or notify bldg control and pay a [huge] inspection fee before starting work, Bldg Regs Part-P july 2005,regards SPARX