Yes, you'll have to check what the laws are for your area. Here is a good guide for the US so you can see what the "norm" is as far as laws go--looks like some are 3 mo. and some are as late as 7 mo.(
http://www.zbirdbrain.com/VAX/RabiesLaws.pdf) Here is something I found for Ontario Canada (
SiriusDog.com - Rabies Immunization for Dogs), which says that dogs must be vaccinated by 3 mo. for rabies.
[Btw txrosegirl, TX's law 4 mo.]
That being said, IF you wanted to wait a bit....maybe til 6 or 7 mo....you definitely CAN'T take her to a dog park, to be groomed or boarded, on a plane, etc, etc... And, not only do they usually not allow it, but you SHOULDN'T simply because if she were to bite someone, even in play, and broke the skin, you would be required to provide proof. If you don't have proof, then they have to euthanize her to test for it!
That is why I personally just go with the law on this one because I do not want that to happen to Bella, even though I can't imagine any situation where she would bite anyone......still.
Bella is only 4.3 lbs. now, and she got her rabies shot when she was 16 weeks old, so she was probably not quite 3 lbs, or right around 3 lbs anyway. I think 2.5-3 lbs should be just fine! (now, those tiny tots that are <2 lbs at that age....or for that matter never get any bigger than 2 lbs!....then I'd be more worried even though I know the tiny ones get the shot too.)