I am sizing illustrator artwork to be specific pixel dimensions (ex: 294 x 76 px). I slice by "making" slice (object/slice/make). I then export by saving for web & devices. When I then preview the artwork I see the file size is 295 x 77px the wrong size by one pixel. all artwork is expanded prior to being sliced so there are no drop-shadows or effects messing with the slices sizing.