Air bleeds are used to correct the fuel curve. Generally you shouldn't have to mess with them too much, if at all.
More important, especially on a Holley 4150 and similar, is the transition slot - which acts as a variable airbleed and a fuel restriction. If its not set within the design range at idle, then the off-idle mixture will be too rich, or too lean (more common) and too slow. On an older traditional Holley - like a 4779 - the working range at idle is .020 to .040" showing beneath the primary throttle blades at idle.
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Many newer cabs may have a tighter range. Definately will if the slots are long or wide. Some are so long or wide that its worth the effort to shorten the slot! I don't think you'll see a need to do that with a 4779, even a newer one.
