I can only speak from experience. I have experimented with attaching
thin metal wires to the top side of my waxes which were long enough
to extend above the flask. Once the investment was solidified I
pulled the wires out leaving air passages from the wax. I cast these
flasks without any vacuum or centrifugal assist. The castings came
out just as solid as if they had been done with assisted casting.
The only problem was the castings had short silver wires where the
air vents were present.
While vacuum casting, I have seen air bubbles come to the top of the
sprue while I was pouring the metal. This made me wonder if the
vacuum method was doing any good in these cases. The castings showed
no signs of any defects.
I have talked to casters who claim they do not use vacuum or
centrifugal methods to cast their work. Would their process work for
all castings? Probably not.
As has been stated several times in previous posts there are just
too many variables to make general statements.
If something works for you go with it.