Roosterlorn said: Another is that some crosses will produce seed which contain chemical inhibitors/toxins that attack the embryo during germination and kill it.
A little nitpicky, but I don't want anyone to think that germination inhibitors kill germinating seed. When present, they
prevent germination, but do not kill the seed. Their evolutionary purpose is to prevent germination at the wrong time of the season, and
allow germination at the proper time in its natural climate. This is a good thing in the wild, but can cause some consternation as we try to sprout seed artificially.
Toxins (and other incompatibilities) can kill germinating seeds.
So now that my fastidious mind is put to rest, this distinction does not change Lorn's or Øystein's advice:
the possible removal of toxins
and inhibitors through soaking is still a good thing. (Myself, I've never heard of a lily seed with a hard shell that
needed to be soaked because of it.) I might add that it is best to avoid soaking in tap water (that would include the inevitable residual softener salt and other contaminants added by your municipality). Try to use distilled water, rain water (or snowmelt), or well water.
Remember that seeds need oxygen to germinate and grow. If you can't plant after a 3 day soak, it is best to transfer them to a wet paper towel in a baggie or sealed container where they can breath but remain fully moist. I can't say how important this is with lily seed in particular, but with many germinating seeds the absence of oxygen causes different (and bad) reactions to occur.