You are correct, it should never be perfectly straight but it never will be. As you drive and the wheels go up and down that should be enough so the joint moves and the grease circulates and the bearings move. As important as the angle is, the length is also very important as a previous poster mentioned. Too short or too long and you will put a lot of stress on the joints and the suspension and eventually something is going to give. At ride height the joint should be "neutral" and at the ends of travel it should be about equal as to length and angle. This is just one of the things anyone doing a swap must consider and work out. Mess up and parts fail prematurely.