What do the instructions say? Typically they will state exactly how much variation can be in the subfloor surface.

Yes, my concrete subfloor was also not quite level enough, and this was spotted by contractor once we had the carpet up. However, he judged it to be pretty borderline, and I did not want to add the delay of sanding concrete, so we went ahead. There are a few spots where there are tiny gaps between sides of boards (which we sealed) or tiny steepling between ends of boards. The problem areas are very few though, and I have no regrets with the choice to go ahead.