When you feel that its ok. I would say not until u find the sex out because at the anatomy scan you will find out a lot. Like about the heart if all organs are in place if little one is developing correctly. It just put my mind at ease and I was able to shop more freely.
When all the summer clothes go on sale you will want to hit thoes up!! Buy for all sizes, like about the size the will be then, i'm due next month and when all the winter sales went on this spring i got all his clothes for this winter/early spring its a good thing to do for saving money!!
I am also due in December (the 1st) and I'm waiting til I know the sex to buy any clothes. Most of what's in stores now is all summery stuff anyway.
Usually after 12 weeks is okay most people will say when ever but as long as you're past 12 weeks you should be okay(: