I was asked this question many times over the last few years, and I never had a definive answer for the asker of the question.
Indeed, it's common that faster yellow plate novices will ask for, and be granted, the right to practice with the white plate experts, but it's far less common to be a white plate expert and routinely practice with the yellow plate novices. I know very well the practice groups and how they are split up. If you race a big bike (600cc and up multi), and especially if you race some classes that aren't combined (no expert or novice distinction), you should be practicing with the white plate experts, especially if you want to go faster as the faster folks will help you go faster. If you don't feel comfortable being an expert and practicing with the white plate experts, then maybe take a step back to yellow?
For the new racers in our club, and for those that asked but never got an answer that satisfied, please explain.