I usually mention it during my phone call to each actor during prep week, and give them the heads up that we are looking to do a straight six for first rehearsal. Obviously they don't have to tell you how they would vote, but I have found that most of the time they are forthcoming if they have an objection.
I am always prepared for the fact that on the first day of rehearsal, they might vote against it, and then we would have to either do a 5-hr day or a full day.
(The only times that I have ever had actors vote against a straight six is if there is nowhere to get food nearby.)