There is a concept that in matters of doubt that pertain to derabanans we pasken lekula. I have seen this applied to doubts in realia as well as halachik doubts (eg machlokes). Nevertheless, it is quite obvious that we do not say this as a blanket statement at all: halacha, at least from after the Gemara's times, does not automatically follow the most meikil rishon, etc.
This is not a modern approach--the Shulchan Aruch/Rema, for example, will often pasken like opinions that are not the most meikil ones. They will often bring multiple opinions down as well, which again implies that we aren't supposed to simply go with the meikil approach.
So: are there any sources that explain how/when to apply safek derabanan lekulah?