I'm sorry but that kind of logic sounds absolutely bizarre to me. You're trying to qualify someone who undercuts by an arbitrary amount of gil as someone who is putting in immense thought and effort into the process or marketing as though there were some magic number that entices people into buying a product. That may be the case for rare, expensive items and even new items, but not for products that sell regularly on a daily basis and suffer no reduced sales rate whether or not the price is 20k higher or 20k lower than the current, cheapest listing. Undercutting isn't just about reducing the price of an item until people start buying, it's to stay at the front of the line so long as you are paying attention. If you're busy, disinterested or simply offline then you are not actively making an effort and that's where other people take their turn to advertise their item above yours. If you listed an item for X price, checked on some others and then realised that your first item was undercut before you even left the retainer/marketing area, are you really going to leave it as is? In most cases, the answer will be "no" because you want to take the chance to push yourself to the front again. So what do you do? Do you choose to take another chunk out of the item value to make yourself the cheapest listing just to satisfy some illusion of playing courteous to your fellow salesman? You can, but that's your choice. Alternatively you can drop the value marginally and meet your same objective without sacrificing anything at all, and then, if the other person happens to notice that you undercut them in the same fashion you did, they can return the favour. In both cases, both parties fairly made their own attempts at being at the front of the listings without needlessly reducing the overall value of the product.
If someone lists an item for a price that is way above what an item normally sells for and experience tells you that it won't sell, then by all means undercut to the point that item falls within a reasonable price-range, but if every single person cut items by thousands of gil at a time because it satisfied some sense of "politeness" then everyone would be profiting a lot less overall and that is not in the best interests of a recurring marketer.
Now, if you want to talk to me about listing multiple listing of a non-stackable item in the market then I will agree with you there. While I undercut by marginal amounts, I don't list more than two items at a time because that would feel selfish to me. If both my items sell while I am away then I feel it's only fair to let other listing have a chance at selling their items until I actually return and bother with checking on my retainer. The price, however, is completely irrelevant in my mind. If you're going to undercut then you're going to undercut. The amount is meaningless to the other sellers because their item still won't sell before yours at the end of the day. If I sacrifice some extra profits just to appear polite then I am only risking steeper reductions on the market board sooner rather than later when the big undercutters come around and make their rounds and take their turn pushing ahead in front of me while I am not watching the markets.