I ripped mine out (s15) for collie, then when I went to put them back in, I couldn't get the seatbelts to come out... Seats were in tho. Got stickered one night for bald tyres and had to go over. Rather than f**k around with the seatbelts, I figured I'd go the 2 seat option so that if I needed to pull the seats out again, I could, legally.
ALL bolts/brackets/anything else sharp/not a part of the body needs to be removed and you need something to block the boot from the cabin (I used MDF drilled into the body). I think that's to do with fumes/possibility of explosion, but I'm not sure.
My car is still rego'd for 4 seats, but have the permit for no rear seats. So in the event I want to take the whole family out, I can throw it all back together (with seatbelts) and still have 4 people in the car legally. In the event I want to go to collie and load up 10 tyres, I can rip it all apart and do that legally.
Only thing is, I think you'll find if you have the back seat in, you will need to have the seatbelts in. So even if you have the permit and get pulled over, not having seatbelts for seats is a yellow, where as if the whole lot wasn't there you'd be fine.