Since the standard UN thread is 60 degrees, and your tool is ground to 60 degrees, moving it along a 29.5 degree line into the cut throws the majority of the cut on the leading (Left) edge of the tool. Many people take this route believing that the cutting force is primarily on that edge and the trailing (Right) edge only cuts the 0.5 degrees left from the compound movement angle, yielding a better finish. This could be true since you can do better in some material with a positive rake, and you really can't practically grind both leading and trailing edges to cut with positive rake. The more positive the leading is, the more negative the trailing edge. If you are using a neutral rake tool, there really isn't as much difference, IMO. It is a point that many will argue, but straight in feed, 29.5 degree feed in.....they both have advantages that are sometimes determined by the machine and material.
If you are feeding in straight, with the cross slide as you mentioned, yes, the 29.5 degrees is immaterial since you won't be moving the compound. In that case, the chipload would be fairly equal on both flanks of the thread, depending on the grind of the tool. Make sure you are understanding that the 29.5 degrees is from the face of the part, not the axis. Not all compounds are marked from the same reference.
