>OK, my thoughts were if collapsing over say 'longitude', you may have a different number of longitude points at different latitudes and so you might expect to provide a weight but I think I'm over-thinking this

I had the same thoughts by the way. There are many complex ways in which you might end up needing weights, but I would hope that people who find themselves in this situation should know what to do, we certainly can't second guess them all.

As the idea here is specifically to avoid simple errors caused by not using area weights I think what you suggest is adequate.