Whether to pre-book depends on the airport, your arrival time, and your tolerance for hassle. At major international airports in most countries, walk-up options are perfectly reliable: official taxi ranks, express trains, and ride-share pickup zones all run steadily through the day, and pre-booking buys convenience more than necessity.
Pre-booking earns its keep in a few situations. Landing late at night when trains have stopped and taxi queues thin out. Traveling with kids, lots of luggage, or needing a child seat, which street taxis rarely carry. Arriving somewhere known for taxi scams, where a fixed-price booked car with your name on a sign removes all negotiation. And smaller regional airports, where the taxi rank can literally be empty after the last flight.
Hotel shuttles are worth asking about since some are free, but confirm the schedule in writing. If you do book privately, use a company with real reviews, confirm the total price includes tolls and waiting time, and know that legitimate drivers wait in the arrivals hall, not soliciting you at the gate. Anyone approaching you inside the terminal offering a taxi is best politely refused.