I would do whatever looks natural and symmetrical

Sometimes this means doing a soffit. If you don't need the extra 8 inches on the other side, it may look more symmetrical and natural to do the same height of cabinets on both sides and the small soffit on both sides.
It's hard to say for sure without seeing the actual plans.

Soffits are often needed on the projects I work on because the houses are mostly row houses, and ducts need to be run against the joist direction. It seems counter intuitive but sometimes it makes sense to have more length to the overall soffit to "connect" it to something or a soffit where you don't technically need it for balance.

A planned soffit can disappear better sometimes, even if it's bigger. than an incomplete "bump" somewhere that the eyes go to and tell you that there is a pipe or duct that needed to be covered up.

We wanted to get rid of all of our soffits, but wound up having to keep one out of three. You can check out the final result in my kitchen pics. The soffit is "disguised" as best as possible on the wall above the sink. It houses major plumbing for the upstairs bathrooms. We ordered the cabinets before we knew the deal, so we had to figure out what would look best. Good luck!

Here is a link that might be useful: Dusty's kitchen