Question About Creases In Stamps

Hi All -
Hoping for some guidance on this issue, seems to be happening with a sizable percentage of the stamps in the shoebox of mostly loose stamps I have inherited. The attached photo shows an example of the back side of stamps with parallel creases about 1 cm apart. In some cases there is a sort of grid where similar parallel creases meet at nearly right angles.

-On a scale of 1 to 10, how impactful is this defect to an otherwise good quality specimen's potential value? Lets say with 1 being 'no impact' and 10 being 'renders it worthless'
-How best to describe this condition so as to be forthright to a prospective purchaser?
-Is this sort of condition common or unusual?

The example in the photo is a block of Scott #804

Many thanks for your input!
-Geoff

These so-called gum ridges are normal on these stamps. If I remember correctly they are there to keep the paper from curling up.

Peter

Thank you for that information, Peter! I was worried that this was a result of the way they had been stored (which was not exactly ideal.) I have since moved all to glassine envelopes and am storing them so they are standing on their edge.
Best Regards,
Geoff

This may / may not be pertinent.
FYI
Gum Breakers on Rotary Printings

Franklin O. Davis feels that the various types of gum breakers used
at the Bureau to keep stamps from rolling up after issue, are of
considerable interest and I agree, except that I haven't found the
time to keep up with them, beyond noting that there have been several
varieties used in the spacing of these breakers. He reports that he
recently acquired a block of the current 20¢ Special Delivery stamps
with breakers in both horizontal and vertical directions on the same
block. This is something unusual that I haven't seen and if
deliberate and not the result of accident, it means extra processing
work for the Bureau since the sheets must be put through the rollers
twice.


- George B. Sloane
Sloane's Column
Stamps
April 13, 1957


https://stampuoso.com/gum-breakers-u-s-stamps

Thanks so much for the additional information - - explains a lot of what I am seeing very well. Also in the small world department, I believe the author of that article was my former neighbor's father, who I remember being told was a noted philatelist!