It's very important that you follow grainlines when laying out and cutting pieces for garments.  Every pattern piece is either marked with a line that should lie on a lengthwise or crosswise grain, or has a straight edge or fold line that is placed on a grainline.  When a piece is cut off-grain by more than a 1/2" or so, the piece will not hang properly - it will want to stretch out of shape because it is slightly on the bias.  Crosswise grain may have a bit of give, but will not stretch like bias (at an angle to the grainline).  This means the fabric itself should be straight before you cut. (See June 25, 2012 entry).