Should you always apply the rules of significant figures?

[lLovePhysics]

Should you apply the rules of significant figures when you are doing any kind of physics, chemistry, or mathematics computation? Like, for example if you are doing homework problems in your textbook, should you still follow the sig fig rules or should you only apply them in labs and measurements?

 

[Office_Shredder]

For physics and chemistry, you should be using significant figures in your homework problems too. Why wouldn't you?