I get that a lot too. Don't think its alignment related, most likely has to do with the car's body roll. Try getting a strut bar, makes the ride more stiff not to mention it also helps with cornering. Also try lowering the center of gravity meaning lowering springs, think that helps too.
Try checking to see if all wheels are properly inflated. I had that issue too and once I put some air on one of the tires it went away. As for Strut bar it will totally be up to you, it's all about preference. I personally will like to get a DC Strut bar, just cause it looks beefier. But if you want something cheaper you can go for the UR (Ultra Racing) strut bar, it's about 30-50 bucks cheaper and I hear good things about it.
Versa's are hard to keep in the lane. They like to wander and I read about it alot online from drivers. People say it due to the high roof, high ground clearance, skinny wheels and steering. I noticed it as soon as I got a Versa. I like you thought there was something wrong until I read up on it. Not saying there isn't anything wrong but the Versa will never feel as planted as some other cars IMHO.