No, it's not acceptable to require a user to edit an auto-generated tweet just to fit the 140 character limit but users should still have the option to review & edit the auto-generated tweet.
Disregarding all best practices & guidelines, the common sense of great experience says, "If you are automating something for the user but letting them approve in the final step, make sure the auto-generated content is compliant with all requirements to minimize user involvement."
The general social media marketing guidelines suggest that all tweets should be ideally no more than 125 characters to allow for old-style RTs (where RT & username are included in the body reducing the available characters for the person who retweets). They also suggest that auto-generated tweets should make sense to readers (i.e. try to avoid truncating words).
So the overall best practice for auto-generated tweets is being concise/short, making sense, and requiring little user editing unless they want something custom. And those who want a custom message can easily modify it because that's the way Tweet This" button works.
The case of answer announcements on Stack:
The purpose of the tweet is to inform your "following" of a question you answered so that they would come visit the site, browse it, and maybe join the community. The key parts in the tweet are the question and the link to it. In addition, the tweet contains a "lead" with the name of the forum and subject & predicate to make a sentence.
Of these three parts, 2 are under total control of admins: the lead and the URL. And the question title isn't controllable. However, admins can also run a query against the database and find out the average character count on all questions ever submitted.
With all this data in their hands, admins need to write a lead that is as short as possible while making sense and put the URL through a shortener (stock like t.co/bit.ly or a custom one) so that the remaining characters of the tweet would fit an average question plus at least 1 standard deviation (Six Sigma might be too tough here). They can also save 7 characters by truncating the protocol declaration
http:// from the URL as Twitter will understand & hyperlink it anyway.
I responded to a UX Q: Should a Tweet button yield a suggested Tweet
over 140 characters? stkx.ge/1234567
I responded to Sci-Fi Q: Is there any indication that latinum is useful in production?
I responded to coding Q: load external html into a div replaces content of original page
with external html stkx.ge/1234567
I responded to English lang Q: Is this sentence correct - “They want to eat food they know
is good for them.”? stkx.ge/1234567