Bronze and silvers are only useful for progression before you acquire gold moves. Eventually you'll find yourself drowning in a sea of gold moves after playing long enough, and there would be no point to hold on to bronze and silver ones at all.
But in the meantime, keep the necessary bronze moves until you get a silver version, same goes for silver to gold.
Moist pretty much covered everything. Until you've got enough Golds to fill your main teams, you can hold onto any Bronze/Silvers you like. Once you've got enough golds, sell literally everything that's not gold.
At higher levels, once you've got a ridiculous selection of golds you can really start to focus on only keeping gold specials/blockbusters that have good stat boosts, but that's a while off so no need to worry about it till you're neck deep in golds.
You can safely sell basically all your taunts at any rarity though
Actually taunts can be very useful depending on the fighter. Especially if the fighter's signature ability works better with a taunt than without one.
Scarlet Viper benefits greatly from equipping the Scarlet Ladies taunt, which grants her regen to remove the bleed she applies on herself.
Number One works well with equipping Aroo Ready? Taunt because it grants 1 hype and the haste necessary for him to gain Enrage on hit.
Armed Forces is better off with Medici Muscle to gain armor and tank blockbuster attacks, in addition to her defensive SA that can grant armor and unflinching. Similarly, Resonant Evil is better off with using Bagpipe Blues.
some bursts are good because they have a 100% proc rate of useful debuffs (in particular painwheel's burst goes well with her aggressive nature and can be fit into her combos), but most bursts and taunts are less useful than straight damage abilities