Is there any reason to keep monster trophies around that you don't want to use?
Like an NPC later that buys them or anything?
Looking for ways to clear inventory weight :/

Well if you don't wish to turn them in or sell them,then just dump.

Turn them in? How?
I mean, I finish the contracts and everything, but they stay with you and give you buffs when equipped...
I'm wanting to sell all but the one I use, to free up space, just want to confirm they have no other special use down the line.

What do you mean by "turn them in"? Do you just mean to finish the quest by talking to the quest giver? Or is there someone to actually turn trophies in to like in Witcher 1?

The only special use is what you said,I doubt there would be two special uses.
If you haven't collected a reward for them,then wait until you find the right person
with quest.

There is a merchant in Novigrad that will give you full price for any item you want to sell. I.E. - Say the trophy is valued at 150 crowns, most merchants will offer you a meager 20 crowns or so. This guy will give you the full 150.

Other than that they just give stat bonus, sell em for crowns, you are a Witcher after all. =)
Whatever you do don't dump em, the Griffin trophy is worth 250 crowns!

Edit: This should tell you what you need to know

http://www.gamersheroes.com/game-guides/where-to-turn-in-monster-trophies-in-the-witcher-3/

Ah, awesome.
Will hold on to em until I get to that guy then, thanks.

No problem! Glad to help.

Also, as far as I know there is no official way to turn them in like the Royal Huntsman in TW1.

Thanks for the tips:)