I agree with that being a useful default with most people, and reliable with even those who you don't know well enough to figure out how they'd react to criticism.

I'd put a bit more emphasis on how putting a white lie into the initial encouragement can cause issues though. If you've said something generally encouraging or picked out some positive, but not actually said anything which you think of as untrue then if they do explicitly ask for a critique then you can give them your opinions and suggestions in full. If you used what you hoped would be a white lie then you must either contradict your previous encouragement or withhold parts of your opinion even if the person genuinely requests it and wants feedback, both of which seem like bad options.