From what I've noticed, it's always one of the singles.
Usually the second one.
The only exception was Clocks being the third single for A Rush of Blood to the Head (the second being The Scientist, which I consider to be equally as popular).
I think they intentionally plan to release their second single as what they believe will be their most positively received song by the public.
Speaking of singles, didn't they say they pushed back Charlie Brown in place of Princess of China? The only reason I can imagine why they'd do this is for timing, which kind of adds to my theory above.