Actually, it sort of depends - on VHF, you are usually going through a TNC that has open squelch detect - or at least the ones I use do (as do the radios I use - channel busy detect)
We have it fairly well standardized up here in the area, and the packet channels are (mostly) empty - in fact, until we started pushhing a few practice nets with ARES, they were totally dead - on the order of a few transmissions/week
Once we are onto a user node, there is traffic control from the node out to the other nodes (the way flexnet works is it bumps the 1200 baud VHF traffic up to 9600 baud UHF, deconflicts, and sends to the next backbone node). We have PMBOs on both the VHF user side, and on the 9600 baud UHF side to get the data onto the internet as fast as possible
NYC is lucky this way - If I remember right, (I could go look it up) we have 4 active VHF packet node frequencies, and around town, the owners have basically said "These frequencies will be Winlink traffic"
The beauty of using the Flexnet is we are just piggybacking packet traffic on existing packet traffic nodes - no real difference - in fact, the path setup is done the same way