This topic has been covered often. For the most part, most users do not start off with any image retention, but rather they develop it within the first 4-12 months. I had the issue develop slowly but surely, getting worse and worse over time until the point that I brought it into the Apple Store and it was blatantly obvious. I was well within warranty at the point (I think I might've had it for around 6 months at that point) and it was repaired within a few days b/c they didn't have any Samsung panels on hand at the time.
As for the SJA-2 panels, they are supposed to solve the issue (due to a lamination manufacturing error) in most cases, but a very small number will be affected by IR compared to the LG SJA-1 panels. I'd say if you have the SJA-2 model, you most likely won't have to worry about this issue.
Overall, if you have a Samsung panel, you're good and you don't have to worry about anything (except for rare cases, of course). If you have an SJA-1 panel, keep an eye on it and make sure you do one of the many IR tests online before your warranty expires (if you don't have Applecare) so you won't have to pay a very expensive $600-700 panel replacement fee for the IR issue a few months outside of the warranty period. FYI, Apple uses the 'checkerboard' IR test, using white and black only, to test IR issues in the store. As I said above, I'd say most won't have issues with the SJA-2 panel either, but you might just keep an eye on it. Don't be frustrated if you do eventually have the issue b/c if it develops, you will receive a brand new Samsung panel. That is what happened to me and now my laptop is perfect.