Lori, you would have to check the schedule to see what kind of wedging is allowed or disallowed.
In some shows only green styrofoam is allowed and others simply state that it cannot appear above the rim of the container.
My only thought is that a white cotton ball might deter from the overall effect of the entry.
I prefer to use rose foliage or the green styrofoam.
Saran wrap also is easy to use and isn't as noticeable as a white cottonball would be.
JMHO
Meg

I don't think cotton balls would be strong enough for rose stems, also the cotton would soak up the water. Most of the shows here So.Ca. do not allow saran or plastic wrap. We use mostly the green styrofoam or aluminum foil.

Thanks for the input. I find cotton balls (or Saran wrap) so much easier to wrap around a stem, and it holds the bloom very upright. (When I exhibit hostas stems....I use cotton balls primarily.)
Personally, I think styrofoam wedges aren't too attractive. Rose foliage would certainly be a better choice... but usually is not permitted.

Saran wrap is difficult, if not impossible, to see, and so it is very difficult to see if you have a piece of it (perhaps left in the vase from the last show) over the end of your stem. Floral foam (not oasis) works best for me. Some like tin foil.

George

Many shows do not permit foliage as it is difficult to get out of the vase if it goes down in the vase as it quite often does. A green styrofoam wedge pushed down so it does protrude above the edge of the vase is quite unobtrusive and can, with a jerk of the stem, pop right out of the vase when the show is over. Another advantage of the styrofoan over foliage, saran wrap or foil is that the exhibit can be watered in that the water will go right thru it. If you include wedging options in your show schedule, be sure to specify the styrofoam as opposed to the oasis material.