grammar - Is it necessary to put a comma before since clause?

Is it really necessary to put a comma before since clause?

If I omit the comma before since clause in formal writing, will that be taken as a mistake?

We often use as and since when we want to focus more on the result than the reason. As and since are more formal than because. We usually put a comma before since after the main clause:

• [result]I hope they've decided to come as [reason]I wanted to hear about their India trip.


• [result]They're rather expensive, since [reason]they're quite hard to find.

Cambridge Online Dictionary: As, because or since?

We use since as a subordinating conjunction to introduce a subordinate clause. We use it to give a reason for something:

Sean had no reason to take a taxi since his flat was near enough to walk to.

• Since her husband hated holidays so much, she decided to go on her own.


• They couldn't deliver the parcel since no one was there to answer the door.

Cambridge Online Dictionary: Since