if you mean its firing into the back of your seat then its fine. give u a nice back massage with the real low bass! my friend has a ranger and his sub is 2 inches away from the back of his seat. its sounds fine
as long as you have grilles there, no. there's no problem. what you really don't want to block are any ports. blocking or impeding port flow will alter the box tuning. having the seats against the subs is done quite often. especielly in trucks and sedans.