In collecting fields in general, having any sort 
of marking on an object devalues that object, 
regardless of *who* did the marking.

I was listening to the radio, and a woman had a 
verrrrrry rare Elvis record which she bought from 
a record store while he was still unknown. 
Pristine condition, it was worth about $5,000 to 
collectors.

However, her record album wasn't pristine... it 
had been signed by Elvis, who had been doing a 
record-signing that day in the music store she 
had purchased it from. The value of her copy? It 
plummeted to about $1,500.

Granted, though, an object is only worth what 
someone is willing to pay for it. ^_^ And if the 
signature of the director enhances the object in 
someone's mind, it enhances the value of the cel. 
But in general, it's a bad thing to do to your 
cels. ;o)

-Cres