The PSE offers the following in regard to grading early imperfs (link to entire PDF after the following text):
There are two different kinds of imperforate issues. First, there are
the 1847-1856 classic imperforates, Scott Nos. 1-17, and then there are
all the early 20th century imperforates ranging from Scott No. 314 up
through the Farley imperforate Scott No. 771.
The margins for all the imperforates can exist in a huge range of sizes
and can be cut parallel to the design or on a very significant slant. If the
margin sizes of any stamp differ greatly, or are cut on a slant, PSE grades
the centering according to what the stamp would look like if it were cut
(blocked off) in such a way as to maximize its centering appearance. For
example, assume that the left margin is triple that of the right. If it could
be cut down to equal the right margin, and such a move would then yield
a centering grade of 90, then PSE will assign a centering grade of 90.
Generally, collectors prefer imperforate stamps to be rectangular.
Accordingly, if one or more margins have a significant slant, then PSE will
block off part of the margin or margins to optimize the centering, and
assign a grade based on that "idealized" appearance.
Having said that the centering grade of any imperforate is based upon
an optimized appearance, the centering of a Scott No. 11 cannot be
graded in the same way as for a Scott No. 371. For centering standards
for the 1847-1856 classic imperforates, readers are referred to the photographs in the center section of this guide. For these stamps, a picture is
worth a thousand words.
Grading the centering of the early 20th century imperforates is much
like that for their perforated counterparts, but the margin sizes must be
larger for all centering grades of 70 and higher.
Many 20th century imperforate stamps have been cut from multiples
so that they have very large margins. PSE has adopted the following
centering standards for these extremely large, "hand-made" stamps:
Centering Grade Description
100J Must show part of the design on all eight surrounding
stamps, or part of the design of five surrounding stamps
with the fourth margin having a Plate number.
100 Margins cut to the frameline of, but not into the
surrounding stamps. A fourth margin could be from a
sheet edge if at least equal in size to the other three sides.
In the past, PSE has given 98J centering grades to stamps which
show parts of the design of five surrounding stamps, with the fourth
margin being from a sheet edge at least equal in size to the other margins.
PSE has also given that grade to corner margin singles with parts of the
design of the three adjacent stamps showing.
Since such stamps could easily be cut down to make ordinary 100
centering, they will now be graded 100. There will no longer be a 98J
centering grade for these imperforate stamps.
98 All four margins clearly larger than one-half the space
between the stamps on the sheet.
95 All four margins roughly equal to one-half the space
between the stamps on the sheet. PSE does not assign
a 95J centering grade for 20th century imperforates.
90 All four margins generous, but slightly smaller than onehalf the space between the stamps on a sheet. PSE does
not assign a 90J centering grade for these imperforates.
For photograhs of actual stamps, the reader is again referred to the
photographic guide in the center of this bookletfile:///C:/Users/rogdc/Desktop/2009...e_092009.pdf