When do seedlings usually branch out or should I force the branch by pinching off the top growth. I am wondering about Key lime and Clementine seedlings I have.

Based on my experience, I tried branching them out when they were just 8" tall, and what always happen is that one and only one bud will grow, and it continues to grow up as if the seedling was not pinched off. Perhaps wait until the main stem is about 1/4" thick.

Some of my FD seedlings, I just let them be and they branch out like crazy even when they are 6" tall.

I have noticed that on my navel a bud fell off but one just took its place, so I will leave them I want the most possible gowth and pinching back might slow that down. Ill will let it naturally branch out it should eventually. Right?

Yes, eventually but if you want the bush type kind of a tree keep pinching the buds maybe every 3 or 4 nodes. If you want a tall tree let them be.