Does it really matter which measuring cup I use to measure flour?
It really does. You will get a substantially different amount of flour if you measure it in a liquid measuring cup (above, left) and a dry measuring cup (above, right).
For solids like flour, use the solid measure. And while most people just dip the measuring cup into the canister and scoop out the flour, you get a lot more than the recipe calls for that way. (That can make your cakes dry and your sauces too thick). And when you shake the flour from side to side to even out the top, you are further compacting—again, getting much more flour than you need.
It's really just as easy to do it right. Lightly spoon the flour into your dry measuring cup, overfilling it, then level it off with a knife. Seriously, it couldn't be easier, and the benefits of light cakes and perfect sauces are really worth it.
And by the way, if you really want to be precise, you can weigh the flour, which is how professional pastry chefs do it. One cup of all-purpose flour weighs 4.5 ounces.