When I had a NG boiler installed in my house on LI, the circulators were placed on the "hot" side of the system so they always had a supply of air-free water from the boiler. Any returned air went to a riser on the boiler where it was removed with an automatic bleeder. If the circulators are on the more normal "cold" side, as soon as a slug of air hits them, they stop pumping- in that case, you'll need bleeders, or have to purge the air by flushing the baseboard with the feed water supply.

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'83 300D, 126K miles.