Of course, the most important thing is that you 
like the cel you buy.  Often times though, I 
think most collectors start on a more limited 
budget with slightly lower expectations, later on 
we sort of "upgrade".  That doesn't mean you get 
rid of every non "A" cel in your collection.
What happened to me was that sometimes I would 
get a cool cel of a character or even two cels, 
then find an "A" cel that I liked better. I would 
then sell the others to buy the "A".  In some 
cases, I sold cels at a loss, so a lot of the 
time now I wait for the better quality cel 
because it costs less in the end.  Sometimes 
though you can't find that perfect cel, so you 
have to settle for lesser shots- off color, eyes 
closed, etc - or maybe you can't afford the "A" 
cel - but found something you like and can 
afford.  In general, I try not to keep a lot of 
cels, (possible exception is DHY and Nami) and I 
typically like the big pretty open anime eyes- so 
I try not to get more than 2-3 good shots of a 
character - but make sure they are really great 
quality in my opinion- so that way later on when 
I see another great cel of the character I can 
honestly look at the cel I have and say, "You 
know, I like this cel I already own better" and 
pass. 
[This FINALLY HAPPENED to me with an Ayako 
headshot! :) ]
Here's a pretty kickass eyes closed cel.  :)