That's bollocks. Making things up to look sophisticated, is stupidity in it's strongest form
No, the lightsource is always (afaik) placed where it fits the composition best. In a lot of paintings, they use multiple lightsources instead of one (El Greco, Picasso) to attract the attention to the most important parts. In paintings with a strong single lightsource, it's placed where it fits the composition best.
For example Caravaggio.
Lightsource from above:
http://tollelege.files.wordpress.com/2007/12/caravaggio_st_paul.jpg
Lightsource from the left:
http://www.lexscripta.com/graphics/Jerome/caravaggio.jpg
Lightsource from the right:
http://faculty.evansville.edu/rl29/art105/img/caravaggio_stmatthew.jpg
(Or we should assume he changed hands over time from left to right to some sort of upper hand or something like that..
)
Delatour would make another good example.
Light from the left-ish side (but within the painting):
http://www.reproarte.com/files/images/T/tour_georges_de_la/die_reuige_magdalena.jpg
Light from the right-ish side (also withing the painting):
http://philippe.gambette.free.fr/Photos/MuseeBesancon/Georges%20de%20La%20Tour%20-%20Saint%20Joseph%20charpentier.jpg
Both artists worked very strongly with light. And it's not true with either of them. If light was so important to them, they would have adapted their composition to fit with their natural limitation, wouldn't they?
If such a statement was true, paintings would lose a lot of their artvalue.. It would mean that an artist couldn't put his own limitations aside. That would truely be something to cry for..
BTW: That would be funny with Da Vinci, as he switched hands in his life (he lost one of them or something). In that case, you could draw a line between his left hand work and his right hand work... 
God: Behold ye angels, I have created the ass.. Throughout the ages to come men and women shall grab hold of these and shout my name...