I've read a number of times that you really can't cast on the same plane of forward/back strokes, 'cause you'll hit the rod and/or mess up your cast.
I believe (not terribly sure) Lefty Kreh actually puts a slight curve in his forward cast just to avoid the 'line' (YIPES!) of his backward cast: so, straight back cast, and then his forward stroke is a slight outward curve to the right to avoid the line hitting the rod.
As far as weighted nymphs and using sinking tips and split shot, I've always been told that you HAVE to throw a large loop in the line in order to cast it properly.

In short, backward and forward strokes can't occupy the same casting plane. (Make sure your head is not in the area either! Take it from me!HA!)

Practice and find something that works properly for you.