Like many things in golf, there is no right answer. I'm basing my reply on Stan Utley's book, The Art of Putting. Stan Utley himself believes in a slight inside-inside arc. On the other hand, teachers like Dave Pelz endorse a straight-back straight-through stroke. Most tour pros have somewhat of an arc but players like Loren Roberts are really successful with a straight stroke. Unless you practice a lot, a straight stroke is really hard to maintain because it involves a lot of little muscles. An arc is more natural. You don't swing your driver straight-back straight-through. If you prefer to have an arc stroke, make sure to find a toe-balanced putter like the Scotty Cameron Newport. You can make sure it's toe balanced by balancing the putter on your finger. If the toe points 45 degrees down, it is toe-balanced. If the face evenly points straight up, it's face balanced. You might want to read The Art of Putting for more info.