Generally Yes,

If you wish to discuss the plot of a film in UK.MEDIA.DVD then it is common practice to include *Spoiler* or *Possible Spoiler* in the subject of the message and not to mention the plot in the subject.
It is also normal to leave a few blank lines at the top of the message.

This gives anybody who may not have yet seen the item in question a chance to avoid having any key elements spoiled for them by giving them a chance to avoid the message/thread.

E.G.

"Whatever DVD/Film *Spoiler*" would be better then
"Did you see that bit where (etc.)" as the subject bacause it gives nothing away.