You can still be screaming and shaking, but complete that last rep IMO. Not going to failure is not the easy way out as some might think, as a matter of fact keeping yourself on the edge of failure, then rest and repeat will IMO give much better results and more volume.
Mental toughness can be viewed in many ways I guess, I still think ending your workout on a positive, such as an extra rep, and extra few pounds lifted, a new PR will have better long term results that going to failure.
Personally I rather practice to succeed that practice to fail.