I'll start the discussion off with discussing False Needling"

This results when the flow through the fuel jet ( both high speed and low speed) changes even though the cross section area of the jet has NOT changed. This happens because the boundary layer along the surface of the needles and jet walls can change size as the needle and jet move closer or further away from each other. Please note the velocity of the flow is low in the boundary layer and if this layer gets larger (in area) the engine will go lean.

Many better needle designs are what are called single sided. That is it looks like a ramp has been grounded on one side. This allows the body of the needle to pilot in the jet and limits its radial movement. Also the advent of dual fingered needle springs helps keep the needle centered as the engine vibrates. And the barrel style needle holder often with its O ring also helps greatly making the false needling issue a thing of the past (well somewhat). High performance engine manufactures often had this needle profile as an option. Jett, Nelson and Fox are three manufactures that I know of that offer the single side (flat) needle.