If I don't like them immediately I let them hang around for a while and later on when i notice like you did they are sitting on the back shelf I will try a small amount, see if my opinion has changed and if not they go in my 'play around box'. Sometimes that juice ends up being just what I need to spark up or mellow out something else I got. Now and then I get an idea for a flavor mix and that bottle is there to play with and so on. Probably never use it ll up that way unless it ends up as a 50-50 with another bottle but it gets a second life. I have really ended up with some great tasting flavors out of my play box.
I am also a bit leery about giving away my e-liquid even to people I know unless I know for sure I just got it and its fresh, or I am using from the same bottle and share with a friend. Its easy to forget how long its been on the shelf and so even giving it a friend next door worries me. I should start making a practice of dating the bottle when I receive it and then giving away would probably not be a worry.
Some of the liquid I have ordered has a "made" date on the label, others I have I mark them with a sharpie so I can keep up with how long I have had them....I think that vaping expired juice would rank up on the "10 worst things that have happned in my world" list