Direct flash should be avoided when possible, but occasional use is of no harm in my opinion. But when taking flash photos make sure you're taking from a distance because up close the flash effect is terrible.
Some owners who uses flash that I noticed do not direct them to the doll (usually DSLR flash that can be turned to face a different direction). There are also some that uses a cover so the flash isn't very bright (you can possibly tape some tissues to your flash xD i've done that before)
The bright light from the flash will make the resin yellow same like sunlight. So once in a while is okay but if you only take photos with flash, that will be bad.
Some owners will freak out if people use flash on their dolls though, so at dollmeets, please ask the owner first if you want to use flash xD But don't worry too much about the effect, the doll will not turn yellow immediately.