Thanks for your interest !

would cross publishing to Steemit actually hurt your main blog's seo ?

Potentially, we've never actually noticed that. but in theory it might.

If so, would you think it's worth it to put a canonical URL in your blog pointing to the Steemit post as being authoritative, or do you think it just wouldn't matter?

No don't do that, if you put a canonical link on your main blog pointing to steem then you basically nuke your blog saying "steem is the original link, point to them instead of me". What we need going the other way around, canonical and backlinks on steem.

For now there is no support for canonical links on steemit/busy etc. So but I am working on a new standard so it'll be implemented.

For now we'll add a backlink, which will be added in the v1 release.

Edit : missclicked and upvoted myself instead of you ^^'