How careful do you have to be with adding flour for kneading?
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.