In theory it matters not, as very low frequencies are not directional. Some people choose them based on cosmetic appeal. The only consideration that is technically important is to make sure the speaker/port is not blocked. I have a front firing on a pin, and a down firing tucked behind furniture on my A/V system. Audibly, they are the same.

Front firing ones look tacky imo. Down firing blend in better. I do not believe the sound is different. All about size and wattage.