Seems that MathRegion.DrawToBitmap doesn't take care of the Optimization attribute. This is by design?
Example:
MathRegion mr = new MathRegion();
mr.Terms = this.ListValues[n];
mr.Font = this.Font;
mr.Optimization = OptimizationType.Numeric; // <(--- symbolic or numeric nothing change
mr.PrePainter();
mr.DrawToBitmap(e.Graphics, new Point(mr_posX, mr_posY));
mr.Dispose();