well it depends.
do you have dirt and dust in your environment or where the fan intakes air from?
If so, I would say to fix that asap. If you need a carbon filter for the intake that is fine to do, ya know, like how there is a filter on the exhaust taking the growroom air out, you can have a filter for bringing the air into the room too. You may have to build a prefilter box of some sort but get it done.
don't cut corners when you grow.
Imagaint if someone else was growing close to your room. They were growing outdoor from seed, and the male pollen got into the air. That pollen could potientially travel for miles, or a bee could bring it in, then BAM! you have male pollen in your grow room and you cant figure it out. That would have been avoided if you put a filter on the intake and made sure the air quality was good.
Does anyone know any devices to check air quality of a grow room? I know there is a co2 tester. Is that really the best thing you can use to tell how good or 'clean' the air is in your room.