This is correct for thermoplastics that fuse when squished together, such as PLA, as it will eventually iron itself out. However the clumpy thermoplastics such as ABS, TPU and PETG tend to stick to the layer beneath rather than fuse down into it as they work best when deposited not squished/fused. As such the excess is pushed to the sides, which then makes it’s way up when layed down in consecutive lines, such as the top surfaces. As the material can’t go sideways, so it’s forced upward. This causes the nozzle to drag through and pick up those clumps forced upward on the next layer above which results in blobs when it finally drips back off the nozzle as well as the scraping through the layer. I’m sure you’ll appreciate this mechanic leaves an unpleasant surface. Therefore with materials such as this it is imperitive to have the Z Offset correct as this sets the stage for the rest of the print moving upward.
Once I replaced the Z Offset machine start G-Code with that I posted in the Z Offset thread (with exactly the same height adjustment findings as 3DTech’s post incidentally), PETG and ABS now come out pretty flawless (assuming you know the material well enough to have dailed in the rest of the profile also, such as flow rate, temperature and cooling). My results show all surfaces, bottom, top and somewhere inbetween are like untarnished glass. Well, 99% of the time, these materials are still clumpy. This principle I imagine being the same understanding 3DTech was getting at in this post noted above originally, hence the need to adjust the Z Offset.
However, there is good reason BL squish things down well. Bed adhesion. I can imagine their stance being it is better to have good bed adhesion and slightly marred surfaces, than have a model pop off and destroy the hot end. So the caveat here is please be aware of your materials bed adhesion preperties and take appropriate measures to ensure it remains stuck to the bed during printing if you are going to adjust the Z Offset upwards for that material. That said I have had no adhesion issues to date.
Anyway, this is just from my experience over many years now with 3D prinitng (building printers from scratch, upgrading cheap printers, contributing to the Marlin source code and writing out many profiles for Klipper) during which time I have done extensive work on Z Offsets as these principles apply to all printers. I hope it helps some folks.