I keep some of the standard weapon mods around, because for instance, I find an advanced .44 revolver and I'm using a powerful .44. Now, I want the advanced receiver from the gun I found, because I can't make it (need a higher gun nut perk level). So, I switch the advance receiver out with a standard one that I have laying around. That means I don't have to waste any materials crafting the standard receiver. Now, I can attach the advanced receiver to my current gun and then I have a powerful receiver mod, which might be useful later on.
If I see that I keep mods around for a while, I just sell them to a trader/merchant.