For practically any stroke nowadays, a pistol style grip (hands spread more across the racquet grip, index finger a bit higher) is typically used on any groundstroke, slice, forehand, topspin, backhand, flat, etc. combinations.
However, some players who enjoy holding the grip lower than others (such as Federer and Nadal) have a less finger-spread version of the pistol grip on their groundstrokes. It's all up to preference really.
Currently, most pros prefer the pistol grip because it allows them to stay loose while have a more secure connection to the racquet. In practice, you'll eventually find it easier to adopt a pistol grip when you face tougher, spinnier opponents.