The answer is, as always, "it depends."
If you have a good-sized outdoor antenna and the transmitters of all TV stations you want to receive are 10 miles away or so, chances are fair to good that no amp is needed at all.
If the stations are roughly 15-30 miles distant, a distribution amplifier inserted just before the splitter could be warranted.
Beyond about 30 miles in most cases, you'll want an antenna-mounted pre-amplifier to counteract signal loss in the cable between the antenna and splitter. Pre-amplifiers have two components: The amplifier up at the antenna, and a power injector, mounted just before the splitter, which sends DC power up the coaxial downlead even as signals travel in the other direction.
Bear in mind these distances are very general benchmarks for good signal environments -- those without ridges, mountains, heavy forests or big buildings in the way.
Avoid those barrel-shaped inline amps that boost signals "downstream" of the splitter. They're typically so noisy that most actually hurt
DTV reception instead of helping it.