The narrower gage of a 10-speed chain won't shift as well on a 5-6-7-8 cassette / FW, especially on an indexed driveline since it won't be able to pick up the shift ramps until it's 'derailed' much further off of the cog you're shifting out of.
There's still a lot of decent quality 7-8 speed chains out there, and they're still way cheaper than 'outdated' 10-speed.
I don't really have a problem with having a bunch of loose chains laying around the garage. I don't go through them fast enough to keep a stock of new ones on hand; I buy them as-needed. I usually leave my new chains in the box, so it's got a label on it that tells me what it is. Old chains that I keep around for spares/repairs/flips go in a Ziploc bag, so they don't get the rest of my stuff oily. I put a note on the bag with sharpie, (Kid's Bike, Beach Cruiser, Old Ten-speed) so I know what kind of bike it came off of, and what I can use it for.