How careful do you have to be with adding flour for kneading? When you're working a dough (kneading or rolling out), instructions often say to flour the surface you're working on and your rolling pin or hands. This helps the dough to stick less. However, I always am very careful that I don't use too much flour, because I'm afraid this will change the recipe. Is this fear irrational or would I need to use a lot to have an effect on the result? Is there a certain percentage of original amount of flour that you can't surpass?

I'm particularly asking about bread dough, but I think this will also apply to other doughs. I guess it would have more rapidly an effect on wetter doughs in general, but I'm not sure. So feel free to include broader answers.
Don't worry too much about it.

Be careful that some doughs are wet (high hydration - look at my question here), so they tend to stick more and are harder to manage.

If a dough is hard to manage, just let it sit in the fridge for a couple of hours. It will become tougher and easier to handle.

If you keep on adding flour, this will alter the bread formula (affecting the salt content and hydration), an additional percent or two flour won't make much of a difference.