The TSA Pre lane is a separate lane and only there you can not remove belt, shoes, jacket, liquid bag, laptop. You can not exercise your privileges in another lane. Yes, as more and more people are enrolled into Pre sometimes using that lane is just not worth it. Enjoy your part in the security theatre!

If you enter security through the first class line, you will not have access to TSA Pre-Check. The TSA Pre lane is a separate lane and only there you can not remove belt, shoes, jacket, liquid bag, laptop. You can not exercise your privileges in another lane. Yes, as more and more people are enrolled into Pre sometimes using that lane is just not worth it. On the other hand, http://www.outsideonline.com/1957481/why-does-my-tsa-precheck-never-work claims

You'll still be able to leave on your shoes, but will have to bypass PreCheck's additional perks (like leaving your laptop in its bag).

I was unable to find anything on the TSA page confirming this.

Edit: http://www.tsa.gov/tsa-precheck/what-tsa-precheckR clearly says:

If you have a TSA Pre✓® indicator on your boarding pass, proceed to the TSA Pre✓® lane

It is a very interesting question whether you are allowed to use a normal lane if you have the Pre indicator -- I doubt anyone would care as long as you are scanned.