I've reupholstered quite a few sofas and chairs in my time. You still have to measure and cut the same number of pieces of fabric to cover the piece. The difficulty in upholstery (unless it's a chair seat or something like that) is that you have, in some cases, to disassemble the piece to get the thing covered correctly, and to be able to pull the fabric tight enough. Also, an electric heavy-duty staple gun is the best method of applying the fabric, I think. I've done it with a hand-held stapler, and with upholstery nails. It's time consuming and very difficult to do unassisted. And then, if there are loose cushions, you have to sew them anyway. I think it's far easier to make a slipcover for the base of the thing and then make the cushion covers. I don't think it takes more fabric. It may take a tiny bit less, in fact.
So now I measure carefully, cut the pieces to the shape, pin them inside out to the chair, lift it off, bring it to my sewing machine and sew all the seams I've pinned. Usually I do the back of the chair first, depending on how the thing's shaped. Then I put it back on the chair, and add the seat cover and arm covers. Most slipcovers have a skirt, or at least a piece below the seat that runs all the way around the chair. If you cut and pin, you can get a non-wrinkly, close-fitting cover that has the added bonus of being removable for washing or cleaning.
Here's a not-bad site for general instructions:
http://www.denverfabrics.com/pages/s...slipcovers.htm