When you elect for a 180-day waiting period, most contracts will have a 14%-17% discount on premiums. Depending on the size of your contract, you may find that the monthly savings would be worth waiting the extra 90 days in the event of disability.
For more information on physician disability insurance: http://www.www.mddisabilityquotes.com/blog/disability-insurance-protecting-the-goose-that-lay/