The ideal degree of taper for the walls of a cavity preparation is 2-5° because it balances retention and resistance form. A slight taper allows the restoration to be seated properly while minimizing displacement forces. Less than 2° creates issues with seating and leads to poor fit, whereas more than 5° reduces retention by allowing easier removal of the restoration. Thus, a 2-5° taper ensures optimal mechanical interlocking and longevity of the restoration.

Reference: Sturdevant’s Art and Science of Operative Dentistry, 7th Edition.