My recent experience--had the right balljoint replaced and I asked my very trustworthy mechanic the same question. He told me "no" it wasn't necessary to get an alignment (I think he was trying to save me money, believe it or not!) However, I noticed that the front end was shaking badly at 50 MPH +. It just so happened I needed new tires anyway, so while I was at the tire store I had the alignment checked--it was the worst they had seen in quite a while. Now with the new tires, alignment and new ball joint it's in great shape again!