I don't agree, there is already a spoiler prefix that is used, it is assumed that everyone who has watched the episodes have watched the teasers for the following week.
No. I watch online and never get previews. The spoilers are in place to discuss the current week's episode without spoiling for those who haven't watched yet. I don't think future episodes should apply. It's fairly reasonable to ask for warning if someone is going to bring up the previews cos Dexter previews are known for giving away way too much. I remember in Season 4, when Trinity finds out Dexter's real identity and shows up at his work place. That was the biggest thing that happened that episode yet it was in the preview.
You can discuss the previews mate, no problem, just try and give a little warning before you do. I know what you're saying, if it's in spoiler tags then we have no right to complain but we all use spoiler tags in general to merely discuss what has happened in the latest episode. I check spoiler tags expecting to see discussion on the latest episode, not spoilers for future episodes.
I don't see how previews count as part of the latest episode.