Is it worth sacrificing a bishop or knight to stop your opponant castling early in a game?

As above, at my level maybe? Is it a good tactic, as long as I can castle?

Just for castling rights? It's not. That is hope chess, you should try to find good moves. 

There are some befamed openings like the Jerome which involve sacrificing a piece for just that but apart from the memes they are not really known to be solid, or otherwise serve a profitable use. I like to sacrifice material and be down the exchange a lot, but usually my main drive behind making such a sacrifice is to gain a lead in development and piece activity. With piece activity comes more room and more routes to control the game. It's not always worth the exchange, but I'd rather have this than play defensively and let my opponents form a plan more than I do myself. (and sometimes I sac because I'm dumb)

I can only imagine that you're thinking of landing a knight on f7 as white while your opponent is relatively undeveloped...  Fantastic move if you have the knight protected, but (despite having spent more games than I should have playing around with the idea) it's just all kinds of awful.  Preventing castling is not worth a piece.  Maybe if it supports a really specific plan.

As a side note, I personally would much rather sacrifice a piece after my opponent has castled instead of before. Because trying to tear down the wall that's keeping your opponent's king safe is already a much more reasoned incentive to sacrifice than preventing them to castle.

In other words, I'd rather have them castle and only then start going bananas, because its purpose is usually more justified.

No, it's not worth it.

Typically, it's not even worth trading Bishop plus Knight for the opponent's f7-Pawn plus Rook. The main drawback is that your Bishop and Knight represent most of your developed pieces on that sector of the board. What good is it to expose the enemy King if you have no remaining developed pieces nearby to attack it?

Sometimes 2 minor pieces (especially if it is 2 bishops) can be more valuable than a rook and 2 pawns. The thing is, rooks tend to be more valuable towards the end of the game. In the middlegame minor pieces often have bigger role than rooks.

Of course, this is not always the case, but I've seen some game (I think from Hanging Pawns YouTube channel) where he traded 2 minor pieces for a rook and 2 pawns. He thought that he was winning, but it turned out to be a mistake.  He ended up on the defensive, and in the end lost the game. Engine  showed that this transaction was unfavorable for him, even though point wise 7>6.

As for the topic... you should make sure you have something concrete for the piece. At the very least some active, promising position where you believe you will, at the very least be able to get something back for the sacrificed material. 

No