As one whose weight slides up and down, I have been in your position of not knowing for sure if I would need an extender but wanting to be prepared. If you think you may not need an extender, simply go to your seat and discreetly try the belt. If it is too tight, either use the call button or just wait until a flight attendant comes by and flag him/her over. Quietly ask for an extender and they will bring it to you. If you want to avoid even that amount of attention, just quietly ask for one as you board and they will get one to you.

I agree that you seem to be over thinking this and stressing out needlessly. Many people need a few extra inches on the belt and the airlines stock several extenders on board for that reason.