There are specific situations in which plausible ignorance has value. For example: While I presume my house must have lead paint (it's too old not to have had it, and I don't see any evidence it was ever completely stripped), I've never actually tested so if I ever sold it I wouldn't have to declare it. Not that I plan to sell in the next two decades, or that I really think anyone else should have any doubt.
But in general, if you've actively signed something you are presumed to have read, understood, and agreed to it, unless you can make an extremely good case for it having been actively misleading or can show you were coerced into signing.
So I still say "don't sign anything you don't agree to, and don't assume you know what it says"... but I'll accept the addition of "unless you have expert legal advise the the contrary." And I mean a lawyer on your payroll -- no matter how much you trust them, you shouldn't assume you can trust their lawyer.