As a user I prefer feedback on incorrect data input to be as soon as possible, not after I have keyed in a number of other fields. As long as the field validation does an immediate check for the Esc key, bailing out if it was pressed, the user is not trapped in the field with no way out.

And as an end user, I do not want to be nagged incessantly {s}. I prefer to have all the validation occur when I try to save the data and be presented with all the problems at the same time so that I can fix all of them in one fell swoop.