The ASE method is to open the bleeder, this way the old fluid won't be pushed back into the system. It's not as important on older cars, but for those of you who own newer ones with ABS systems it is. It's very important that older contaminated fluid not be pushed back into the HCU for the ABS unit.
I'll let you in on a secret, though, as someone who did all this professionally (ie, made a lot money at the dealerships making sure the customer drove out safely) the master cylinder is proportioned to hold the exact amount of fluid for the entire system. There are only two reasons for the master to ever by low: the pads or shoes are worn out forcing the pistons out to capacity or you have a leak. If you check your master cylinder and it's low don't ever top it off! You have a brake issue that needs to be looked at and fixed. Think of it as having a toothache and rubbing Orgel on it instead of going to the dentist. Yeah, the pain has gone away, but you didn't cure the reason your tooth hurts in the first place.
DOT 3 brake fluid is also prone to moisture. Ever had serious brake frade? I mean like going down a big hill, riding the brake and have the peddle go away completely on you? Some of that is the friction material and the metal getting hot enough to heat sink, but most of that is actually caused by the fluid getting hot enough to boil whatever moisture has condensed into the DOT 3. Two cures for the this: DOT 5, which is almost pure silicone, pricey, and will last forever, or flush your brake system completely every couple of years. Most manufacturers recommend flushing the system every 2 years or 24,000 miles.
