Air rises, so if you want to get the air out, try bleeding the highest radiators. You can get air trapped in any line depending on how level they are, too, so it wouldn't hurt to try the other, lower ones too. If you have some sort of air extraction device (likely) in the system, it will potentially, eventually get the air out unless there is a leak that allows it back in (this is common if an expansion tank fails, when things cool off, a vacuum can result drawing in air). Air in the boiler specifically, may need to be purged, but if you are not getting any more out of it, and have some in the radiators, you'll need to get it out of the radiators. My unprofessional opinion.