Is there any scrappable junk I should hold on to that could otherwise be used for other purposes?
The big one that I try to avoid scrapping is Pre War Money.
Cloth is easy to come by, and Pre-War Money has a high resale value and zero weight, which makes it an ideal way to pull caps out of an ATM machine trade with merchants for a few extra caps whenever you come across one.
There are many scrappable junk items that can be used whole for other purposes, for instance for specific quests or as a crafting ingredient:
Used for Quests (repeatable quests italicized)
- A Biometric Scanner and a Military-Grade Circuit Board are required for the main quest The Molecular Level.
- 4 High-Powered Magnets are needed for the Brotherhood of Steel quest Liberty Reprimed, but they are provided in the Boston Airport workshop.
- Yellow and Blue paint cans - Abbot (Diamond City)
- Bloatfly Gland - Doctor Duff (Diamond City)
- Bag of Fertilizer - Priscilla Penske (Vault 81)
- Adjustable Wrench, Ball-peen Hammer, Combination Wrench, Hammer, Screwdriver, Wrench - Calvin (Vault 81)
- Anti-Freeze Bottle, Gas Canister (for Reactor Coolant) - Proctor Ingram (Prydwen)
Used as Crafting Ingredients
- Abraxo Cleaner - Mentats, Mind Cloud Syringe
- Anti-Freeze Bottle - Berry Mentats, Berserk Syringe, Yellow Belly Syringe
- Baseball - Baseball Grenade
- Bloatfly Gland - Bloatfly Larva Syringe
- Bloodsac - Skeeto Spit
- Brahmin Skull - Mounted Brahmin Skull
- Coffee Tin - Slocum's BuzzBites
- Molerat Teeth - Mounted Molerat Skull
- Pencil - Endangerol Syringe
- Radscropion Stinger - Radscorpion Venom Syringe
- Skull - Spiked Pole
Actually, I've found a handful of items worth holding onto, if only for their caps/weight value. If you currently don't need the resources they provide for crafting.
- pre war money (kinda obvious since its higher value than caps and has no weight)
- gold anything (watch/locket/flip lighter, if you need gold for crafting, scrap the flip lighters and lockets, since the pocket watch doesn't weigh much and has a really high resale value)
- synth components (sell them instead of scrapping them. They sell for 50-100 caps each, while scrapping gets you 2 plastic which is super easy to get from a million other things)
- packets/cartons/boxes of cigarettes & cigars (almost no weight and a decent sale value. they only provide useless cloth if scrapped)
Just a few ideas. basically just compare the weight, scrap components, and sell value to know what is worth picking up/scrapping/selling. I usually dump all of my 'to keep/sell' items on dogmeat as I go (since he's the the only companion that doesn't collect random crap on his own) so when I get back to my settlement I can just dump everything without thinking about it.
Some npcs will trade random junk for caps (like the guy who wants tools) but usually the price isn't worth the time it takes to constantly travel back and forth to drop it off for the petty amount of caps they offer.