Any blooms will become transparent fairly quickly under water ~ the thicker they are, the longer it takes for them to change. It may take longer if they are thirsty first but honestly I would not fill the vases until I was actually at the event. You can still do the designs and weight the flowers with just a small amount of water in the vase and that you can do a week before the event because of the lasting quality of orchids.
This also helps tremendously for hauling the centerpieces to the venue. Just make sure that the vases are not cold or they will condensate.
