From what I've heard, the reasons you give for declining the invitation are extremely important. If you decline and give no reason, for obvious reasons, they'd be reluctant to invite you a second time.
Previous applicants in this situation have said that PC asked them to write a statement when they reapplied about why they declined before, and why their situation is different. When you decline, tell them (in as much detail as you feel comfortable) why you feel the need to decline the invitation. When you reapply, repeat why you declined the previous invitation, and then express how your circumstances have changed. If you are clear with them, I don't see why this should hinder future service.