Follow-up to my question on postage stamps: are cancelled stamps totally worthless?

Judging from the answers I got from my earlier question, I gather that stamps in mint condition are what people want. Can it be said that cancelled stamps are not worth much?
What about cancelled stamps from the 1950’s or 60’s? Foreign stamps from the same era?
The reason I ask is that I’ve been going through my parents’ stuff and there are lot of these.