Some unopened bottles lose volume to evaporation after twenty years, although I think that's rare with modern corks. If you really want to keep those one-dram-down bottles for a long time, you may want to take some measures to be sure of the seal--corks dry out after a while, speeding evaporation. Don't lay the bottles down, though--high alcohol content will eat the cork and spoil the whisky. Just find a better seal. Don't ask me what! And resist the temptation to pop it open every now and then for a sniff.
Most of us here are agreed--once it's open, best to drink it in a reasonable amount of time. Six months to be really safe, two or three years tops, and then only if you are careful.
Closure topics and spoilage have been discussed much here--use the Forum Search function (top of page) to look for "cork" or "spoil" or "deteriorate".