the long accepted general best practice (at least as explained to me) was - as short as possible but no shorter.

so - use the small small cog combination, make the chain the shortest possible length that it is not sagging (that is, the jockey cage is just starting to take tension) and then set the chain at that length. Run thru the various cog / chainwheel combinations and confirm that you have enough chain and that shifting is good. Sometimes too much chain tension will inhibit shifting to a smaller cog. On triple chainrings you may not be able to use the combinations of large ring / large cog.

No one rule covers all possibilities, but this approach yields good results in most cases.

Mark Petry
Bainbridge Island, WA USA