At what stage should acknowledgements be inserted into the paper if review is not double-blind?
If it's not double-blind, then it generally doesn't matter whether the acknowledgements are in the initial version or not: the reviewers will simply ignore them.

When there are page limits involved, however, I like to put the acknowledgements in as early as possible.  That way, their size is built in from the beginning and we don't have to figure out how to fit the extra space during the revision.