My understanding is that in the past,
In the past, these threads on new books with polls, the spoilers were marked with spoiler codes even though the thread was marked (Spoilers!). People were being polite and doing this so major spoilers didn't get seen out in the open.
There's no reason to buck what's been working and start having open spoilers where we didn't used to have them. People who are used to these threads won't be expecting open spoilers. So the best solution (IMHO) is to just keep these sorts of threads the way they've been for the good of everyone.