Jon has mentioned several times that he hardly signs anything. How can we determine what we need to sign and what we do not need to sign?

For example, when purchasing a house or a car, we are asked to sign a ton of paperwork–is it all necessary? How can we determine what is necessary, and what is not?
Also, whenever we sign up for anything, we are asked to sign (often electronically), or agree to terms of service–for example, a class, a cell phone provider, an internet provider, utilities, etc.
It seems like we are asked to sign for everything nowadays.