Any mod can be useful in the right circumstance
As you gain experience you'll start knowing what to keep for using and what sells for lots of money.
Definitely keep mods that have 15%+ in damage, even if it's two different types
...and on-hit mods
...and high elemental attack str mods especially phase and ignite.
...and stat ones because you never know when you need to temporarily pop in a mod so you can use your item until you gain more stats.
...shield pen and shield overload depending on what type of char you have