I've been using 10' cables which were longer than I need. I just made a pair of 5' cables with low mass connectors and love the results.
When you go ultra short there is not much to them, you might consider low mass connections (to do low mass right, you should change out the binding posts on both the amp and the speaker. I use BFA type connectors on my cables. KLEI makes low mass connectors.)
I have a friend who does just what you do, balanced interconnects to his monoblocks and 18" speaker cables. He swears by it.
Jerry