Do it mate. Mines can come in and out no probs. However, one day after a session at Knockers, I left the harness in and have been using it ever since. Mines are 3-point jobs and the strap coming through the hole in the seat, attaches to a bolt on the face of the rear seat, just about the floor. It means that backseat passengers have to straddle it, or I can just take it out. That said there are seldom passangers in the rear. The only downside is that the harness holds you tight to the chair, so reaching forward to change CDs etc. and to check for on coming traffic when situated at a junction, can be a bit of a nuisance - but all you have to do is quickly loosen the belts at the shoulders and life is fine. I love the secure feeling they give and I wouldn't like to go back to seat belts.
On another note. Finally got those DEFIs of yours fitted yesterday - lovely!