Why not just add the index? It future-proofs the performance and, as mentioned in the article, for small datasets, performance is not an issue anyway, so adding an index won't affect the small-dataset performance.
I agree, you don't want performance to suddenly degrade and then you have to find out why. This can be especially bad when this happens with many tables that should have indexes, and performance is sluggish, but not for any specific action.
