Do I need to do a code reading after every merge?
Yes, of course.
Automerge algorithms are helpful, but they're not magic; they just include the changes that were made to both sides of a file if they don't conflict. There's no guarantee that the resultant change compiles or is even not gibberish. There's no guarantee that the logic isn't a complete trainwreck. (Some have speculated that the Heartbleed bug was the result of an automerge changing the logic subtly and was not caught in review.)
This is true of any version control tool that does an automerge (which, presuming you're using something that was written in the last 15 years or so, almost certainly does.) Though this is not to impeach automerge, which resolves two changes to the same file and generally does a fine job; this is also true of a merge in general. If you modify some file A
and I modify some file B
, there's no guarantee that the merge makes sense.
Best practice: you should always review your merges before you commit or push them, even when they automerge successfully.