If you are using one of these systems : Cell Excellens, SP8-IN1 or LSM700, the main CO2 valve is attached to 3 individual ones:
The main valve should always be opened, and you should only touch the individual valve which is controlling the system you booked.
Please do not forget to close the individual CO2 valve when your experiment is finished, but please LET THE MAIN VALVE OPEN: someone may be doing live imaging as well!