Allen---thats an urban legend.
. . . probably promulgated by the folks who make four-bars. A typical street rod doesn't have enough travel to cause problems.

When you choose an axle drop, make sure you consider everything that goes with it: tire/wheel diameter, spring height and attachment point to the frame. Picking the wrong parts leads to either a nose-up attitude - or worse, sparks created by your oil pan.

Take a look at this photo, and you'll understand what I mean

http://members.cox.net/otb_vigilante...9_26/front.jpg

Super Bell makes great axles. They have tube and I-beam.

http://www.peteandjakes.com