Yes you must tell your ball driller exactly what you want. If a hole is being worked out more than a little bit...it's only natural that the top ends up a little larger. If I have a customer that doesn't want any taper, I make sure to oval the hole as much as possible on the press, and then take a little more time making sure I don't focus my attention on the top of the hole.
If you tell your ball driller you don't want any taper, and then it's tapered anyway...time to find a new ball driller.
Little known fact: In Russian "Hope" and "Change" translate to "Tax" and "Spend"