A stamp whose design is well centered within four nearly equal margins is aesthetically more pleasing than one that is "off" on one or two sides. Because of this, stamps with perfect, or near perfect centering have traditionally sold for more money than those that are visibly off center.

As the perforation process occurs after printing, a very slightly misaligned sheet may result in the perforations being closer to the design on one or two sides. Grading the centering of a stamp is complicated by the fact that improving production methods over the past century and a half have resulted in more accurate and precise perforating techniques. Consequently, what may be above average centering for an early issue may be only average or even below average for a modern issue.

Traditional discussions of centering have usually broken stamps into three categories; poorly centered issues (typically 19th century), median-centered issues (early 20th century) and well-centered issues (later 20th century). While this approach is certainly logical and reflective of technological progress, it is flawed in the sense that what may be "Very Fine" for one issue is only "Fine" for another. The expertise needed to know which issues are typically poorly centered is considerable, and confusion often results on the part of the novice, and even the intermediate buyer.

PSE therefore strives to apply only one centering standard for all U.S. stamps with the exception of the first perforated issue of 1857 to 1861 (Scott Nos. 18-38), and their corresponding reprints of 1875 (Scott #41 through #46) and the 5, 24 and 30 issues of 1861 (Scott Nos. 67, 70, 71, 75, 76, 78 and their grilled counterparts). The 1857 stamps were originally designed as imperforates, and because the margins are often virtually non-existent, the perforations are often touching the design even on well-centered examples. Aside from these pre-Civil War issues and the few 1861 issues, the remaining U.S. stamps are evaluated using a single, consistent criterion. In this way, the standard can remain steady, and the listed market values can adjust appropriately.

The logic behind this can be easily understood using a non-philatelic example. Assume for example, a rare antique car is being offered - let's say a 1911 Buick. All the known 1911 Buicks (including this car) are rusty wrecks. But, this particular one is the least rusty, and the best one known. Should the car be described as being in "Excellent" or "Near Mint" condition? Of course not. Simply because it is nice for a 1911 Buick does not change its actual condition. It might accurately be described as "only fair, finest known." Those who want an example of this car will therefore bid accordingly, and the car may realize a handsome price. But its rarity does not affect its grade or condition.

Using a philatelic example, the PSE centering standard for Scott No. 596, a rare and typically poorly centered stamp is consequently the same as that for Scott No. 595, a stamp that comes in the full range of centering. In this way, superb centering is just that, and there is no such thing as a Scott No. 596, "Superb" for issue. The inconsistency and confusion resulting from a "relative" scale is obvious. According to the PSE centering standard, the best-centered Scott No. 596 in existence is only Fine, but the market retail price for this centering can still exceed $100,000.