Does it matter which fist ends up on top?
Generally, your dominant hand should be the one on top when the fists come together at the end of the sign. However, as long as the reaching and grabbing motion is clear and the fists end up stacked, the sign will be easily understood.
Can I use this sign for phrases like 'getting sick' or 'getting tired'?
No, this specific sign is used for physically acquiring, receiving, or obtaining something, like getting a gift or getting a jacket. For changes in state like 'getting sick,' ASL uses the sign 'become' or simply signs 'sick' with an appropriate facial expression.
How fast should I pull my hands back?
The speed can vary based on the context of your sentence. A standard, relaxed pace is best for everyday conversation. If you are urgently grabbing something or emphasizing that you finally got it, you might perform the pulling motion more quickly and sharply.