Very dangerous... if left unattended the wheel can start to flop around and damage your brakes etc. in the process. Worse case is driving on a motorway and the wheel comes off making you suddenly change lanes and that could be a recipe for disaster in traffic conditions. Just keep listening out for rubbing/grinding noise from the wheels every now and then - music off/quiet road and listen to any sounds that may give you concern. Otherwise its real easy to do a basic test.. you could always jack the car up/use axle stands to be safe.. place your hands on the wheel at the 9 o'clock poistion and 3 o'clock position, gently try rocking the wheel as if you were going to take it off.. if there is excess play in the wheel then this could be a sign of potential wheel bearing failure.