The Scott catalog is a guide to stamp values. Ultimately, the market sets the price. But the value of a stamp can vary greatly depending on condition and centering. When comparing stamp values from different dealers, auction houses, etc – make sure you are comparing apples to apples. For example, a copy of Scott #245 with nice balanced margins (VF centering) and no flaws will probably sell near or perhaps over the Scott catalog value. The same stamp where the perforations cut into the design (AVE centering) with minor flaws like missing perfs or a small thin may only sell for a small percentage of catalog value depending on exactly how bad the faults are.
In general, faults that don't detract from the visual appearance of a stamp (such as a shallow thin or light crease) will sell for more than for the same copy with noticeable faults (damaged perfs, heavy crease, or large tear).
Everyone has an opinion on centering. The PSE and PF offer graded certificates on stamps where they list the centering as part of the certificate. The Scott catalog is pretty clear about centering and I personally follow their descriptions. You won't find items on my list marked "VF for this issue." There are a few stamps where the quantity known to exist is small and all (or most) of the copies exhibit poor centering. So you'll see a footnote on that stamp in the Scott catalog saying that the price reflects a particular grade of centering. See Hawaii #1-4 for such a listing.
Some issues are notorious for poor centering. The 1869 Pictorials are one example. The Black Jack (Scott #73) is another issue hard to find with great centering. The Jamestown Exposition issue (Scott #328-330) is difficult to find well centered, particularly Scott #330. Often times, you'll see descriptions for these and other issues as "Very Fine for this issue" which in reality is perhaps Fine or maybe Fine-Very Fine when compared to other issues.
Here is how I grade stamps:
AVE – the perforations cut into the stamp design
F – the perforations touch the stamp design or just barely cut into the frameline
F-VF – the stamp is noticeably off center on one or two sides. But there is clear white space between the stamp design and the perforations.
VF – the stamp is well centered with ample margins. But with the naked eye, you can notice it is slightly off-center on one or two sides.
XF – the stamp is well centered to the naked eye with ample to large margins. But under 10X or stronger magnification, you can measure that the stamp is slightly off center.
Superb – the stamp is well centered with ample to large margins. Under 10X or stronger magnification, you cannot measure that the stamp is off center at all. In other words, absolutely perfect centering all around.