Just out of curiosity, though from what I know from my friend that camera flash can cause our bjd to yellow. I mean, I heard about these advice but in the same time, I do see some bjd owners using camera flash during the photography sessions... So actually, which one is true??

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.

http://energy-liger.tumblr.com/post/33345263955/geekisthecolour-bjdconfessions
http://brittmiscast.tumblr.com/post/34070484663/funnylori-bjdconfessions-i-cringe-when-a

No.
Okay, I correct that-- it's only harmful if you take photos of it every minute of every day for months with flash every time. Yellowing is caused by UV exposure and heat (not necessarily together), and while the flash of a camera emits a bit of both, it's a very very VERY low emission unless your doll is like 2cm away from the flash :'D Plus flashes last for only a fraction of a second as well, so anyone likening it to leaving your doll in the sun for an hour is wrong too :9
Also if the UV emissions from a camera flash is dangerous enough for a doll, it would be harmful to humans too as it would be at a level high enough to cause damage to human skin. I doubt they'd sell cameras with flashes that way!

Saying that though, flash photography is not very flattering if not done right orz Use your flash responsibly, for the sake of the beauty of your photography!