I don't think he's going to sit out games. That's 2 months away. I think we're only hearing about it now because tomorrow is the deadline for a long-term deal. I suspect that if they don't get a deal, then he will play under the tag and try to finagle a deal where the Cowboys agree to not tag him next offseason in exchange for him signing the tender.
Today he has the leverage of threatening sitting out... he's trying to use it to get a long-term deal. After tomorrow, he loses that option, but not the leverage. He can still refuse to sign the tender until the Cowboys agree not to tag him next season. I suspect that he will continue to threaten to sit out unless he gets that agreement... Eventually, if that deal is not conceded by the Cowboys, then he loses some more leverage when the season begins. He starts losing money then, and if he misses more than a certain number of games, then he doesn't get credit for the year, and can be tagged twice more.
Nothing Dez has done yet is cutting his nose off to spite his face. It's all about exerting whatever pressure you can in negotiations to get what you want. He wants a long term deal now... If they don't get a deal, then Thursday he will readjust and start angling towards no tag in 2016... if he can't get that deal, maybe he readjusts.
If he ever gets what he ultimately wants, then he's not cutting his nose off to spite his face. If he just sits out games and doesn't get anything out of it, then yes. But it's too early to say that's what he's doing now.