Eh, I don’t think its quite so dire as all that.
There are important rules in Blades that tell you to hold on lightly, keep the conversation open, empower the players, etc. etc. etc.
The example in the book of “Oh dang, I want to do that is it too late?” “Nah, go ahead and do it now” is an embodiment of those rules, even if they “contradict” the rules about setting effect before the roll.
It’s a problem when it becomes a problem in play – not in the abstract.
If it becomes a problem in play (as Stras points out it could) then you should talk about it, get serious about procedure, or whatever. But until then, it’s okay (even better!) to be flexible.
Don’t try to play the game perfectly to avoid friction. Instead, understand that there will be mistakes and friction, judgment calls that have to change later, etc. – and create a play environment where that’s low stress and fun. Over time, you’ll get better and better, and the learning process will be collaborative and fun, instead of stressful and punitive.
The rules of Blades in the Dark aren’t just conveying a set of proper procedures to apply the mechanics. They’re also conveying a certain approach to developing a more smoothly functioning game group. The role of judgments and corrections – and the way the group can deal with them without legalistic precedence citing (or other bad habits) is crucial in this development.
Aaaaanyway… don’t adjust effect after the roll.