It's not normal to have to add brake fluid due to a low level in the master cylinder. Typically when you replace the pads you would ensure the fluid was at maximum level. As the pads wear, the level drops, but if there are no leaks, it will not drop below the minimum level. I would never add fluid if there wasn't a leak, and if there was, I would take care of the leak and add fluid according to the wear of the pads.
And if you do, the next time you go to replace your pads and you push the pistons in to make room for them, you're going to overflow your reservoirs when the excess fluid backs up into them.
-RP