When is it appropriate to use the idiom "various and sundry"

Sundry is defined as "including many things of different kinds." It's second meaning "an indeterminate number" is assumed by its first. Many things isn't a "determinate number." Since the first definition stands for both, you're better off dropping various. There will be sundry opinions about this I am sure, but they will no doubt be of both "an indeterminate number" and "differing in some respects."

"Various and sundry" is in fact not redundant. Various means varied, and sundry means of indeterminate number. Proper use of this idiom is as an adjectival phrase denoting both of these concepts, for example, "He added various and sundry ingredients to the stew."

No one bothered to answer the original question: What is the proper use of the idiom? It's perfectly acceptable to use this construction, meaning a (large and) diverse group of people or things, in creative writing. In fact, I'm about to use a variation of it myself in a detective novel: "assorted and sundry suspects"

Pleonasm

Fowler notes that many pleonastic set phrases were created (not originally created) to achieve emphasis, but because of overuse they now invariably wind up “boring rather than striking the hearer.” Many of these—such as any and all; fit and proper; aid and abet; save and except; sole and exclusive; null and void; terms and conditions; cease and desist; and various and sundry—have been adopted from legal jargon. Other common pleonastic twins that usage authorities find objectionable include if and when; unless and until; compare and contrast (from educationese); first and foremost; and the much-despised each and every. The prudent copyeditor will completely eradicate such clichéd pairs.