I've heard over and over that when it comes to baking, measurements cannot be ignored, and you need to be very precise. This question covers how precise a measurement of flour should be, for example. But if getting your baking just right requires being so painstaking in measuring ingredients, how is it possible that all of the amounts in recipes come out to such neat and easy numbers? I can't remember if I've ever seen a flour measurement go more specific than the nearest 1/4 cup. The smaller chemicals like baking powders are usually to the half tsp, that I've seen, but I can imagine they go down to 1/4 or 1/8. And eggs basically always come in ones, or one yolk, but there's not much you can do about that. Weight measurements are more specific of course, but have you ever seen a ratio like 4.2683 oz of flour per egg?  How accurate is this really? Does this mean if your flour measurement is off by 1/16th cup you'll be just fine? Or it just won't look the same as the one made by the person who invented the recipe? I have a hard time believing that with such complex chemistry involved, the optimal quantities are so close to large fractions of our units of measure. Is there actually a bit more fudge room with some ingredients than we're being told?  Also, if so, which ingredients are more forgiving? I'd guess that baking powders are among the least. 
I've always been taught that baking is a science when it is compared to cooking.  Cooking is very much 'to taste' and very individual.  There are not as many things that can go wrong with a standard recipe in cooking, and you have a lot more room for creativity.  You don't have to look at baking as that precise.  However, unlike cooking, where you can add or subtract from a recipe with no real harm to physical structure, that does not hold in baking.  For examlple, if you feel that there is too much salt in a recipe, cutting back can (and most likely will) have a cascading effect through the ingredient chain. You have to understand your ingredients and the effect they have on other ingredients.  That is what makes baking a more precise science. And in terms of flour, it is often the most 'ranged' ingredient.  Depending on flour type, miller, altitude, water temp. etc., the amount of flour in a given recipe is always a guide.  Again, you have to know your ingredients.  You will never see 'one and an eight cup plus 2 TBSP hard flour' in a recipe, because it is so variable for many different reasons. Flour and water are the two most flexible ingredients, and are always variable. Hopefully this helps, I am sure someone will come along with a more scientific explanation for all different ingredients, I am just offering an experience based answer to your question. Also, you are right, do not mess around with baking soda and powder.  If you do, you are asking for trouble. I would suppose that the rule of thumb would be that the more exacting an ingredient is (tsp, quarter tsp), you don't want to change much. 