I don't think you have to, but some people do it anyway. I did it because I think it lets people know that the build thread is part of the WBO and what category it's in.

Not everyone viewing the forums will be going directly to the WBO area and will just be looking at 'New Posts' or whatever. Adding 'WBO 2018 Vintage' or 'WBO 2018 Scratch Built' to your thread title alerts people to the fact that the build is part of the build off.