Not all cars with an airbag has pre-tensioners.Air bags here are designed to work with the seatbelt that's why the seatbelt has a pyrotechnic pre-tensioner that's designed to pull you back into the seat as the airbag fires.
My Zafira handbook (which I can't find now to check) does say you should try and stay far away (no distances given) from the PASSENGER airbag becuase it is huge. IIRC this is emphasised for carrying kids in the front.
On a personal note, I'm fairly tall, but I drive with the seat back pretty straight up, which means I'm normally pretty close to the steering wheel airbag. I crashed my car several years ago (not too badly) as I was heading mostly sideways for a kerb I remember thinking "Please don't let the airbag go off, please don't let the airbag go off". I just hope it only causes less damage than it prevents. (Actually I hope never to see it )