You can declare it, but keep in mind that "Dream sequence" isn't going to be put on the screen with a title card.
When will the audience know it's a dream? Requiem for a dream simply plays them like that's what is happening, then cuts back to show the dream never happened (girl and guy sitting at a table. girl stabs guy's hand. Cut to: guys hand resting on her hand and her looking at him) Vanilla Sky had dialouge to tell you.
Either of RR's examples will work. There is no hard and fast formatting rule. Just be sure it is clear to your reader. Once you've had a couple people take a look at it, you'll know whether they followed it or not.
Since I pulled it (not word for word because that's stealing) from the Screenwriter's Bible I'm pretty sure it's correct. Use which one you like best but use it consistently. Don't make the dream squence one way then the fastasy another way, then the flashback another way. They're all formetted the same, just pick one you like and keep it consistent.
It's all about giving the reader the same roadmap as the audience without over dictating or over writing right?
So if the audience should know it's a dream sequence i.e., the director clues them in somehow like the squiggly screen and tinkly sound effect on Gilligan's Island or Happy Days (for example) then go ahead and declare dream scene. You've given the reader the same nod that the audience will get without dictating the directorial choice that will indicate it to them.
If we as audience or reader should have no idea that it is a dream sequence then obv. NCJE is spot on. no need to declare it and that is probably appropriate more often than the cheesy examples I mentioned.
EDIT: Just reread the 1st post and am reminded that the poster was describing the "wakes up from a dream" scenario. So NCJE is right.