It will depend on the function, For making bulk bowls for functional use then there are ways of extracting more bowls per log, both covered in the 2 answers above mine, but for making bowls that are singular and more form than function wood wastage is a part of the plan as you are only looking for the pattern you wish to portray. Straight grain efficiency in my opinion is best left for the industrial machine, hand turners excel when the chosen piece of timber is difficult, frustrating, and has a high risk of failure, best results are when only a fellow wood turner will understand how many hours it took to achieve the apparent simplicity of the finished piece.