In the past I would have moved the blot 3, then off with the 6. However a recent opponent claimed that under the rules I am obliged to use as much of each throw as I can, therefore would have to take the 5 off, and leave a blot on the 2-point. As long as you legally play the entire roll you can choose which die to move first. This bothered me and I checked the Monte Carlo rules and found it was not so. I have just found your rules in Bibafax 37 and they agree with the Monte Carlo ones.
Many players take up the cube and polish it ready for action.
In an important match a player might quote you and insist on the double, thus creating an awkward situation for all concerned. You would then call me over and you'd have to explain your action regarding the cube. One must be very careful when considering using the cube and I am very surprised at your being allowed to change your mind. My advice, to keep things straightforward, is not to touch the cube unless you mean to use it. Players that seek an advantage by 'testing the water' by reaching for or picking up the cube must be discouraged from doing so.