Let the edge shape guide the choice
Straight-grain strips work well for many quilts with straight edges. Bias strips are cut diagonally across the fabric and have more give, which helps them follow curves. The right choice also depends on the fabric and the finish you want, so follow your pattern when it specifies a method.
Know what the binding calculator covers
QuiltMargins works out binding made from straight-grain strips cut across a usable fabric width. It counts the strips and allows for the joins and finishing tail. Selecting diagonal joins changes how those strips are joined; it doesn't turn the cutting plan into bias binding.
Plan bias cuts separately
For bias binding, use instructions for the bias-cutting method you intend to sew. The starting fabric shape, diagonal cuts and joins affect how much usable binding you'll get. Keep the quilt perimeter as a useful starting measurement, but don't use the straight-strip yardage as a complete bias-binding purchase amount.