The co2 canister is simply to keep dispense pressure up. The sugar you prime with will create enough co2 pressure to dispense with initially. I'd pump the co2 in there if you're noticing the keg is starting to glug or if you're leaving the keg half empty for a while. That said, if it's a party and the keg starts to glug but you expect to finish the keg before the evening is out, just unscrew the cap and let gravity do its job.