This paper (presented at our 1997 Conference) by Warren Kuhfeld of SAS Institute is a good overview of design principles for traditional conjoint. Kuhfeld introduces the concept of design efficiency and argues that orthogonality in conjoint experiments was generally required in days when computers were not widely available. If an orthogonal design was used, relatively simple formulas were available for hand or calculator ANOVA computations. Today, general linear models such as OLS do not require orthogonality for the unbiased estimation of effects.
Warren explains the principles of orthogonality and balance, introduces the measure of D-efficiency, and compares two computerized search routines for finding efficient experimental designs: SAS's PROC OPTEX procedure, and Sawtooth Software's CVA designer. For the size of designs commonly used in conjoint experiments, Warren found the CVA routine to find designs about 97% as efficient as OPTEX, but that CVA's designs tended to be more balanced. He also found CVA easier to use than OPTEX. Warren concluded: "For small problems like you would typically encounter in a full-profile conjoint study, CVA seems to do an excellent job. However, for larger and more difficult problems, it often fails to find more efficient designs that can be found with PROC OPTEX."