I don't treat census records as gospel, not only for the reasons you gave, but also for the fact that the census takers themselves were also a bit questionable. Plus you never really know who gave the census taker the information. In this case however, it actually helped me zero in on more exact info.
Likewise, death certificates are great for the date of death, but the rest I like to have another document to back them up. After all, that is one document where you know for a fact that the person who actually knew the answers wasn't filling out the form.
The older the people are, the less credence I give to the census. I have one ancestor who over the course of her lifetime shaved 15 years off of her age.