Joining of NAB will surely affect your preparation for CSS exams, but i will advise to join the NAB and do preparation with job....

Because CSS is more of gambling to one career, its upon your luck whether you qualify with little preparation and fail even after a lot of hard work......

I am telling this thing from my experience as I have appeared for CSS exams twice and one time i get failed and next time even after qualifying i was not allocated because of huge competition in Punjab and limited seats.....

And i have left a lot of good opportunities just for the preparation of CSS.... whereas i have interacted many of the candidates of CSS who did there preparation with job and they qualified.....

So i will advised to all that not to miss the opportunity of NAB or any other just because one is considering to prepare for CSS......