Bob,
That advice is not universally true. It depends on the battery type and charger brand. Battery Tender recommends mounting the charger as far away from the battery as the DC leads allow. I suspect this is to avoid corrosion from gassing, but that would not normally occur if using a sealed AGM battery for instance.
I am a few weeks away from mounting my charger and am 52% sure I will mount it in the location shown in the hymnal.