As far as I’m aware perfumes will go off once you crack them open, so to speak. There is a guideline use by on the packaging somewhere and, although I’m not suggesting you bin it if it’s out of date, just be aware that the smell can alter in an almost imperceptible manner due to the slow and subtle breakdown of the different molecules involved due to heat and light (store in a cool dark place, obviously). I’m sure Globalti can add to this or even dispel it as myth perpetuated by the perfumeries.
I’m not saying they all turn nasty, some may become more to your liking, others not so much…