Back when I was a new MM2, I had a friend who was an FT1 instructor at BEE School. He got selected for FTC. By this time he had been in for a lot of years, ans was close to retirement. He wanted to retire as a CPO, so he took the promotion, bought the Khakis, carried that stupid book for weeks, and was promptly told that he would be transferred to a ship to finish his time.
His wife told him that it was time to start a family, which she would not do if he was going back to sea. It was pretty much an ultimatum since she was in her mid thirties at the time.
He decided not to extend his enlistment, went back to being an FT1, and was harassed about it until he retired.
He was out of pocket for all the new uniforms that he could only sell for cents on the dollar.
When it was my turn to take the MMC exam, I simply declined. With only 11 months left to EAOS, and only a marginal chance of being promoted anyway, I didn't feel that it would be right for me to compete for a promotion that I had no intention of taking, and that could cost me money and headaches like it did to my pal.
That was a turning point in my career. I should have taken the test and booted it instead of not taking it. The CO was close to having a stroke as he was yelling at me. Nothing good happened to me for the rest of my time in the Navy.
My advice is to do what is best for you. If somebody puts a test in front of you, take it. If you happen to pass, you have an option open to you that would not otherwise be there. If you don't piss off your CO, he probably won't frock you if you ask him not to. If they do frock you anyway, (which they will do to give you incentive to re-enlist - figuring that you will change your mind once you get the feel of being a chief) you can always pick up some uniforms at the thrift shop or buy them from some poor sap like my friend above.
Yeah, you're taking a billet from somebody else, but it is someone who does not qualify for it as well as you do. Basically, the billet is yours if they offer it to you, and being yours you can do with it as you wish. It is the way the Navy wants it to be. Otherwise, they would do it differently.
I used to think differently. Because they CAN screw with you. But they can screw with you either way. They can only screw with you until they sign your discharge.