[Jeffy]

You should absolutely let the teams know that you are looking to pick them before hand.
What I have always done is on Saturday morning I go around to all of the teams on our pick list, let them know approximately where they seed on our list (ie. first or second pick) and inquire about their robot (struggles and successes), and then see how they feel about being part of an alliance with our robot.

This year I went around to every team, took a measurement and then let the team know whether they would be invited based on triple balance possibility or not. Unfortunately, the stars didn't align in the picking for us to triple.