Is it negative to defrag a minimally fragmented drive?
No, if you intend to invest the moment doing that it's great. Usually you will not get a lot for the time you invest doing it, however.
Remember that the 2% statistics, nonetheless, does not inform you anything. Does that mean that just 2% of documents are fragmented, or that 2% of the drive room has documents pieces, or another thing?
For sure metrics a 2% fragmented drive could in fact be reducing you down dramatically relying on your use pattern.
Still, most of the times the moment it requires to defragment it is significantly longer than the moment you'll conserve as soon as it's defragmented.