I've been using X for a very long time now, which means that over the years I've built up a large and ornate set of X resources, a fair number of which are now obsolete. I recently totally overhauled how my X session loads my X resources, and in the process I went through all of them to cut out things that I didn't need any more. I did this first on my home machine, then copied much of the work over to my office machine; my settings are almost identical on the two machines anyway, and I didn't feel like doing all of the painstaking reform pass a second time.

Xterm has many ornate pieces, one of which is an elaborate system for customizing character classes for double click word selection. The default xterm behavior for this is finely tuned for use on Unix machines; for example, it considers each portion of a path to be a separate word, letting you easily select out one part of it. Way back in Fedora Core 4, the people packaging xterm decided to change this behavior to one that is allegedly more useful in double-clicking on a URL. I found this infuriating and promptly changed it back by setting the XTerm*charClass X resource to xterm's normal default, and all was good. Or rather I changed it on my work machine only, because Fedora apparently rethought the change so rapidly that I never needed to set the charClass resource on my home machine.

(My old grumblings suggest that perhaps I skipped Fedora Core 4 entirely on my own machines, and only had to deal with it on other machines that I ran X programs on. This is foreshadowing.)

When I was reforming my X resources, I noticed this difference between home and work and as a consequence dropped the charClass setting on my work machine because clearly it wasn't necessary any more. Neatening up my X resources and deleting obsolete things was the whole point, after all. Then today I started a remote xterm on an Ubuntu 16.04 machine, typed a command line involving a full path, wanted to select a filename out of the path, and my double-click selected the whole thing. That's when I was pointedly reminded that default X resources for X programs are host specific. Your explicitly set X resources are held by the server, so all programs on all hosts will see them and use them, but if you don't set some resource the program will go look in its resource file on the host it's running on. There is no guarantee that the XTerm resource file on Fedora is the same as the XTerm resource file on Ubuntu, and indeed they're not.

(It turns out that way back in 2006, Debian added a patch to do this to their packaging of xterm 208. They and Ubuntu seem to have been faithfully carrying it forward ever since. You can find it mentioned in things like this version of the Debian package changelog.)

In summary, just because some X resource settings are unnecessary on one machine doesn't mean that they're unnecessary on all machines. If they're necessary anywhere, you need to set them in as X resources even if it's redundant for most machines. You may even want to explicitly set some X resources as a precaution; if you really care about some default behavior happening, explicitly setting the resource is a guard against someone (like Debian) getting cute and surprising you someday.

(There's another use of setting X resources to the default values that the program would use anyway, but it's slightly tricky and perhaps not a good idea, so it's for another entry.)

The reason I never had this problem at home, despite not setting the XTerm*charClass resource, is that I almost never use remote X programs at home, especially not xterm. Instead I start a local xterm and run ssh in it, because in practice that's faster and often more reliable (or at least it was). If I run a remote xterm on an Ubuntu machine from home, I have the problem there too, and so I should probably set XTerm*charClass at home just in case.

PS: To add to the fun of checking this stuff, different systems keep the default X resource files in different places. On Fedora you find them in /usr/share/X11/app-defaults, on Ubuntu and probably Debian they're in /etc/X11/app-defaults, and on FreeBSD you want to look in at least /usr/local/lib/X11/app-defaults.

(On OmniOS and other Illumos based systems it's going to depend on where you installed xterm from, since it's not part of the base OS and there are multiple additional package sources and even package systems that all put things in different places. I recommend using find, which is honestly how I found out most of these hiding places even on Linux and FreeBSD.)