For what it's worth, I have 5 kegs and no manifold. I have never kept the gas connected to my kegs unless I was dispensing for a party. I simply charge my kegs to 30 psi for 48 hours. Then, I drop the pressure to about 12 to 13 psi, and I charge my kegs once or twice a week. This is enough pressure to typically dispense a few glasses of beer without needing to recharge, and then I simply put the gas on for a minute to charge the keg back up.
Bottom line is that you don't need to keep the gas open to a keg in order for it to get carbonated. If you are shooting for some precise CO2 pressure on a keg, then you may want to consider getting a manifold. I haven't ever seen the need for it though.