Well, its best if you do. But it is not needed. The overal diameter of the 19's tires might be different than what you had on previously.
It is up to you tho.
My suggestion is to put the wheels on the car, drive a little and see if the car is pulling or not going straight. If it does any of that, get an alignment done. If none of those occur, you should be ok, but still the alignment maybe off slightly which MAY lead to uneven tire wear.