In theory that is a great idea and one I've used in the past... in practice I have had basses that have put on 0.5kg between being weighed and delivered to me.
I have a 'custom' bass that the manufacturer/luthier quoted the weight of on their website (bearing in mind this is custom order, one of a kind), the bass is a full kilo heavier than they stated! Private sellers are worse; I've asked for 'accurate' weights (going to the extreme of specifying kitchen digital scales not bathroom etc) and yet basses still put weight on during shipping. Though one of our very own BCers was genuinely shocked when he weighed his bass (which he though had to be under 9lb) and apologised profusely before we had concluded our deal.
I'm also still interested in what models are downsized.