I'm just wondering is it wise to delete all post revisions and autosaves from database every once in a while?
I mean they take a lot of database space and apparently have no use after the post has been published.
It all depends on your workflow. If you have no use for the post revisions and autosaves then disable them in wp-config and WordPress will clean them up on its own. You may also prefer to limit how many you keep at once.
Also the space they take up is unlikely to be of a great impact itself unless you're running on a tiny tiny server
Don't go deleting them manually through the database.