I have played around with the posting rules, done some tweaks here and there, but am now getting into a project where I will be altering quite a few rules.
When altering built in functions, is it best to create a new function and switch the rule to use that? Or is it better to just alter the built in function?
I am definitely no expert, but I would think that you want to create a new one. That way, if they update the posting rules, and specifically that function, yours won’t be overwritten.
I’ve noticed on some of our other customizations, the customizations get kept and we cannot see the updated rules unless we view the Base Std revision.
That sounds correct as that is the way Epicor “wants” upgrades to work. I am just overly cautious and prefer to create something new if I can. 
Suggest not to alter the built in function, as, they may be overwritten with new upgrades. You can create new functions and use them in rule Customization.