We applied about 7-8 months before our GCs expired. They expired before we were processed. We took advantage of the 6 month rule.
My wife's interview was 5 days after her GC expired and her oath was exactly one month after the expiration. We did nothing about it and all was well.
My interview was descheduled. I scheduled an infoPass right after I found out that I had been descheduled to find out what was going on. I pointed out my expired GC and they stamped an I-551 (I think that's the number) in my passport extending my GC 7.5 months (yes, a very weird number, but it was about right - I had less than a month left on it when I took the oath).
My daughter's GC expired at the same time. She didn't "apply for an N-400" (being a minor). We were worried about her status, so we filed for a replacement GC. She got her FP about the time her GC expired (at which time they applied a hologram sticker extending her GC 6 months). Her new GC showed up the day before my wife took the oath (at which point my daughter became a USC). Interestingly, until very recently, my daughter had 2 valid green cards and a US Passport (the "extension sticker" expired before Christmas).
So, if your application/GC situation fits the 6 month rule, don't worry about it. They didn't bat an eye at either of our interviews or at either of our oath ceremonies.