Simply..YES! you do need fresh air coming in at all times, wether co2 or not, air air is essental to the plants growth.
And with LZ's Quote: No air going out, you will pass out when you go in, well something like that . simply if you dont have the co2 circulating with fresh air, 100% co2 is there with no where to go, resulting in passing out!
with veg, you dont need filters, just need fresh air coming in, and stale, hot air going out.
flowering, you just need the one filter inside or outside the room (does'nt mater) taking the smelly air out, so to speak.
MAke sure that there is always air moving (circular fan) in our room, not only doe's the plant like it, but it also gives strength to the stems for some heavy bud support.
If you have the tent sussed for the temps, all you have to worry about is the watering/nutes/PH/EC, air flow and alot of love!
hope it helps and makes sence, cause im a bit pissed
but im sure you get the idea