Well after you make a few messes, you remove a little brake fluid before you push the pistons in. But I usually do a brake flush at every brake service now. Topping up is ok, most cars have a brake fluid level sensor that will tell you you need a little esp if it has ABS plus you might have a brake fluid LEAK in the system and you dont really want to run out of fluid. So just keep it in between the max and min level or about 1/2 in below the top if there are no max/min indicators.

 

...I never top off my fluid. Low fluid tells me there is something wrong. It may need pads/shoes. Or, there is something leaking.