it's not much harder to check an asymmetric frame than it is a symmetric frame. The Surly Pugsley is offset 17.5mm so the chain can clear the tires. Walt of Waltworks has built frames with offset rear triangles to get less dish in the rear wheels. Here's an example

I'm going to guess that there are tons of tandems out there with offset rear wheels and people are perfectly happy with them. Plenty of evidence that the industry disregards a few mm. Most framebuilders don't, but that doesn't mean it has any effect.