Cutting the end of the scion to shape, just cutting a wedge is all i have done in the past on apple trees and some grafts work some dont, i havent got much variegated lemon to mess with ( it was £2 on ebay.... ) and i want to put it on a decent size tree and have more chance of it takeing

a program on TV the other day showed grafting and on all grafts after cutting the wedge shape he showed every time taking the knife down each side on the outer bark edges to widen the cambium layer to about 2mm wide on each side on the outside of the scion, removing the outer bark outer skin at the very point
Is it important to widen the exposed cambium layer at each side? ( i never have, but i only get about 30% grafts that work )?

When grafting, you are trying to physically match the cut surfaces, then callus growth unites them, and the scion begins to grow. Areas of intact bark on the scion peice, such as the uncut sides of the v may be partially covered by callus, but dont 'grow' into the graft union. Any exposed cambium on the scion, needs a corresponding cambium area on the stock with which to unite.
Callus can grow to meet unmatched areas, such as when the scion is thinner than the stock, and only fits up on one side.
Grafting of different species, may require changes to technique, timing, or even avoidance of some methods, if the resultant plants are not satisfactory, in the long term.
Widening the cut surfaces, may be useful in some cases, but not others ?
Was the TV show on general grafting, or a specific type of plant ?

I suggest you do chip buds. I've had much better success than with cleft grafts and you only need one bud, no bark slipping required either.