Originally Posted by
carpediemracing
... Using the small-small method is very safe and eliminates accidental wheel popping out (or worse) when you inadvertently use a wheel with a larger large cog (heck, the second cog may be bigger than your current big cog).
This method is only safe if you're sure that the rear derailleur you're using has a wrap capacity that's sufficient for your rings and cassettes. That's true on most bikes, but on one of mine the crankset was changed to one with wider spacing and the derailleur wrap capacity is no longer sufficient.
If I put on a chain using the small-small method it'll end up too short if the bike is shifted to the large-large combination. There would be the danger of the chain jamming and damaging the bike or possibly causing a crash. Using the large-large method of sizing the chain on this bike results in a longer chain but one that will have significant slack if the bike is ever shifted to one of the smallest cogs when on the small inner chain ring. Not ideal but not something that causes any harm. Since the inner chain ring on this bike is only used as a bail-out granny gear on very steep climbs it is only used in combination with the larger cogs on the cassette and in that case there's plenty of chain tension.
So on that bike the small-small method is not at all a safe one to use and I always size the chain using the large-large combination instead.