Do SMD resistors have to be reflowed black side up, or is black side down okay?
There are several reasons to put the alumina (white) side down. For small production/hobby runs it may not matter.


You can tell if the resistor has been overloaded for whatever reason as the passivation/element color will generally change under high continuous heat.
Depending on the technology, the contact dimensions may not be the same on the top as on the bottom.  Pad geometry assumes alumina side is placed into the paste.
It's a little easier to clean which can be an issue in high resistance components.  Passivation layers typically have varying heights/roughness, unlike the flat alumina base.
Wear and tear under mechanical stress, passivation layers are not as tough as the alumina substrate so if the board rubs against it it will eventually get damaged (usually only a problem on thin board under mechanical loads).