Lighting is a touchy subject, because there's no 
one "best" method. Light is harmful, but you need 
light in order to see things.

The cons of natural lighting are obvious--- it's 
an immense source of heat and UV radiation. In a 
windowless room, though, it's not an option, so 
that makes things easier.

The cons of fluorescent lighting is that it 
produces more UV rays than incandescent lighting 
does. However, it's a cooler bulb. The majority 
of the UV rays can be blocked by UV-protected 
glass, which would fall under the standard list 
of archival materials to frame your cels with, 
along with acid-free matting and everything else.

The cons of incandescent bulbs are that, while 
they don't produce as much UV radiation as a 
fluorescent bulb does, they generate much more 
heat. Even if you keep the room temperature 
stable, it's still going to be a stress on the 
object over a period of time. 

The cel has many components to take into 
consideration--- the plastic in the acetate, the 
paint, the lines. Given the wrong set of 
conditions, the plastic is probably going to 
deteriorate much more quickly than the lines will 
fade. So it was more of general advice for the 
cel as an entirety, rather than specific advice 
for the lines in particular, as no one wants a 
cel with perfect lines and yellowed plastic. :o)