i had the same question when i got a co2 set up. the answer is no. mj plants use co2 in the day and oxygen at nite. i vent my room just before the lights go out. check out my grow http://forum.grasscity.com/general-indoor-growing/531815-1000w-mh-hps-lst-scrog-dwc-first-grow.html
I too am doin CO2 for the first time. Here is some great info on CO2: Quick Grow South Articles also, this is a calculated to determine the how to use your CO2 system- www.hydroponics.net/learn/co2__calculator.asp