I was always taught to slowly add the acid to the water. Dilution of most concentrated acids is a highly exothermic reaction (releases heat). Adding the acid to the water distributes the heat in a large volume of water and if the temperature gets high enough to locally boil it gives off steam and perhaps some dilute acid splatters. Adding the water to the acid results in the heat being concentrated in a smaller volume of acid so it will typically heat up faster and any splatters will be of a more concentrated acid.
It is also important to start with cold or at least cool water and pay attention to the temperature of the mixture. This is absolutely critical when diluting very concentrated acids by a small amount - for example 50% water and 50% acid. It is less critical for our typical horticultural uses where we start with a less concentrated acid and often dilute by a factor of 20:1 or even higher. If for some reason you need a hot acid solution dilute the acid and then heat it. Do not start with hot water.
BTW the same precautions apply to diluting concentrated bases such as lye (caustic soda).