Yes, some do that but there's usually a second fan which expels that air from the case -- it also serves the other components.
As I say, look at the computer as a whole and use your judgement.
My concern is a conversation with a friend who took the side off his computer and was adamant that this helped as the heat sensor on the case fan showed a lower temperature !!
some sensors are not accurate haha .. my compaq pc Second fan is designed and built just beside the cpu heat sink ( heat sink has its own fan) on the casing and it is thermal controlled by the mobo. but it does not serve as a heat expelling fan for the gpu instead mainly for cpu and the surrounding components . furthermore the Stock Psu that came with this pc BLOW air out of the casing ( omg rite ?) . So It really made me regret buying a branded com