I always fill out my immigration cards/forms before landing. Why, you ask? Because it will help you get processed quicker. I normally fill them out right after the cabin crew distributes them so I don’t have to worry about it later. After I complete the card/form, I always put it together with my passport and then put it away so I don’t lose it.
I have traveled quite a few times internationally and noticed that quite a few people who doesn’t fill it on prior to reaching immigration and then get sent back to the end of the line. It will also help if you have the name and address of the hotel you will be staying at. Some countries are okay with just the name but some will require name, address and telephone number. It will be best to be prepared.
So do yourself a favor, just take a few minutes to fill out your immigration card/form when it is distributed and then you won’t have to worry about it until you reach immigration.