Big-big is the more conservative approach, particularly if you set it using the largest cog you will ever use on that chain. Small-small assumes you have a rear derailleur capable of wraping up the entire tooth count and if you do, it does mean big-big will work too.
The problem with small-small is if you have a total-tooth count setup greater than your rear derailleur can handle. For example, I routinely substitute a 26T granny for the OEM 30T granny on road triple cranksets and that gives me a total tooth count greater than the rd will handle. I set the chain at big-big and realize it will go slack in small-small but that not damaging and not a problem. Not being able to handle bib-big can do a lot of damage if an inadvertant shift to that combination is tried.