Conway argues that leaving off the parens from user-defined subroutines makes them harder to distinguish from built-ins.

That might, just, possibly be the case for those very few heroic martyrs who insist on doing everything in green&white and eschewing the benefits of syntax highlighting.

But they are a rare breed indeed. For the rest of us, depending upon our needs and taste, built-ins can be made to stand out--or not--as we so chose.

Were this a truly important distinction for me (or anyone using my editor) to make, then I could have built-ins appear double-bold fluorescent pink letters on a day-glo yellow background. I might even be able to make them flash, though I'm not sure because I've never thought to try.

But I don't, and I doubt you'll find many who would. My argument with this particular piece of "best practice" is not that the argument is wrong, but that it is the wrong argument. Why is it important to make that distinction?

User functions are just as important as built-ins; but no more so. Besides which, most built-ins can be overridden by loaded modules, and many are to good effect. What benefit this distinction then?

Given the effects that book has had, I'm kinda surprised that anyone is still promoting it.