The reason you want a ported vacuum on the distributor is so you can eliminate any pulsing the vacuum adds to the timing --- so if you're vacuum is bouncing around a bit at idle -- then so is your timing...
Having said that - that's not 100% pure -- because it "depends" -- depends on how much pulsing your vacuum is doing etc... but basically that's the reasoning behind it.
The other thing is -- that if you time your initial - without vacuum advance -- and you've adjusted your idle screws etc -- and THEN plug in vacuum and advance the timing -- all your other adjustments are now worthless. Timing changes with RPMS -- idle changes - and on down the line.
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