For me, it would totally depend on the purpose for showing the dog. If you are just showing for sport, then why not? If you have intentions of breeding, especially if it's a bitch, then I would wait for one that will make atleast 5lbs. Like was said before, males are more acceptable to be small.

Showing is a sport whether it be pure for the fun of it or for the show casing of our breed to the best representation. However, my thoughts are this....while I totally have fun showing because I love to do it (or I wouldn't be doing it LOL) I still believe when you go in the ring it needs to be with "the best" representation of the breed and in my opinion a small bitch under 4 1/2 pounds at adult weight, I'm not talking puppy :) should be in a loving pet home or on your couch like my sweet Krissy. When I realized she wasn't going to grow anymore and I realized it wasn't wise and really a waste of money dragging her around because finding judges that like "really" small yorkies was hard to find wasn't in Krissy's best interest (btw she was and still is just 3 1/2 pounds). Although she loved showing and was a spitfire she's much happier being our little lap princess! Could I have finished her "yes" was it that important "no" was it money wasted "in my opinion yes". I'm not looking for how many champions I can get. It's the best representation of our beloved breed I try to bring to the ring.
My decision, my opinions :)
Anyway the orginal OP never mentioned whether it was a dog or bitch LOL. A smaller male like Mary said is alittle more accepted in the show ring than a bitch because judges do look at a bitch for breeding purposes not all but most.
Anyway this post is not meant to sway anyone or make anyone feel bad but only my opinion as a show exhibitor and breeder and what I would do and how I think :) In no way does it mean you can't show what one wants.

Donna Bird
Brooklynn's Yorkshire Terriers

under 4 pounds is not a fault there are some judges that like small but they are few and far between it would be very hard to finish your dog