Cross-chaining on a 3x6 system -- how bad for wear?
I would not be concerned in the short term, say 6 months to a year. There are several things in your favor. A six speed chain is wider than a modern 10 speed chain. Wider equals more material equals longer wear. On a MTBSO parts are used that are cheap. Cheap means steel instead of lighter alloys. So I think you will see some wear but it will be minimum.