There would be no vote buttons if you were not allowed to upvote or downvote; you can see the rules in-game by clicking the save button and then "Save Upload Rules", or on the forums.
Sorry, if you look at the rules (Section G: Rule 3) you will see asking for votes of any kind is frowned upon, but it should be clarified that asking for an absence of votes in either direction is also not allowed.
No, asking people not to downvote also falls under rule G3. Basically, any mention of votes is against the rules.
If it's not intrusive, I'll sometimes leave the save up and simply ask the author to remove the message. Asking for upvotes will immediately get your save unpublished though.