My experience "hoarding" parts of any sort (buying well before I need them) is getting stuck with un-used parts I no longer need. Things change (my bike, my frame, my preferences) and it only takes one part that I spent money on that I can't use to offset any money I may have saved by hoarding. I've got some really nice unused 9sp MTB stuff that I have absolutely no use for right now
So not I just keep extra tubes, pads, and maybe a chain around. that's it.