Bert van den Berg wrote:

Is it true, however, that you always need a reverse blocking diode on all solar panels that don't have one in-built?

Maybe on some panel types, but I'm with you, 33milliamps leakage from a
40 watt pair of CIS panels into a 500ah bank. Installing blocking
diodes would be a loser because of forward voltage drop when the panels
are high noon producing with near fully charged batteries. People who
say always install blocking diodes paint with too broad a brush.

Blocking diodes are a good idea when constructing higher voltage
series-parallel strings, to prevent damage to shadowed panels.