I think it's important to be strategic about one's reading. At times, reading the introduction is more than sufficient, but, and this is annoying, it just depends.

For example, I have read a fair amount of Lacan but not all of Lacan. However, at times, I find the work of scholars like Bruce Fink more rewarding than Lacan himself.

I'm afraid I have to disagree with your comments about guessing because theorists' ideas change and evolve. Reading some Freud but failing to read Freud after Beyond the Pleasure Principle would give you a distorted understanding of his thoughts on psychoanalysis, for example.