(getting loot whenever it is convenient since the Dryden brothers can upgrade weapons and armour - for a considerable commission)
While true, waiting until one gets to 18th or so will save a lot in gold, if one the PC; hence ,y suggestion for getting as high as possible. Plus, for me it makes a better connection to the story to wait.
True, but leaving the three canonical DLCs until very late - so that the loot is fully levelled - has the inconvenient side effect that you have to do most of the treaty missions without that stuff. Spirit Healers may covet the Boots of Casting One More Spell (Warden Commander boots, Keep) and Cailan's Arms (RtO), for example, and there's a lot of other useful kit.
With the Dryden brothers' complicity and a bit of cash the loot scaling problem can be eliminated, and the money can be recovered by brewing potent lyrium pots for sale. Same principle for generic weapons and armour.
That would make Cadash Thaig pretty much the only mission where loot quality (level) might influence the decision about when to do it.
@ Mike: just checked by making a save on Bann Loren's lands, so that I could play with 'runscript levelparty' before transitioning to Ostagar. Cailan's kit was red steel up to level 17, silverite for 18 and 19, and dragonbone from level 20 on. Some of the generic loot went to dragonbone earlier. However, these results are not necessarily representative since the loot levelling might involve a random fudge factor here, as is the case with merchants.