The key to using your badges well is not to follow some prescribed order of puchases, but to take a serious look at what your badges will get you and what you will lose by making the upgrades.
There is no pat answer that is right for everyone.
Make yourself a spreadsheet. List out all the stats for each upgrade piece, and badge cost. Calculate for each piece the actual change from your current stats. (For example, if you current legs have 55 stam and you upgrade to the badge legs you are getting a real change of +23 stamina).
Now, calculate what that cost in badges for each one of those points of stamina are! 100 badges / 23 = 4.6 badges per point of stamina. Do the same for every stat.
Look at your weaknesses. Is your dodge low? Are you miles below the hit cap? Do you ahve very little expertise?
Now, balance your needs, the cost per point of various abilities for each piece, the total cost in badges, and your likely near-future upgrades and make an informed decission about what it is you should spend your badges on in your specific case.
As an example, the auto-blocker is a GREAT trinket if you are having threat problems because you find yourself under-geared compared to your DPS. I got one early and am very glad I did because it helped me hold threat in 5-mans, and that helped me gather more badges faster. Now, that took into consideration that I almost always do the daily. If you rarely do 5-mans, then that reasoning doesn't apply to you.
Another example, at one point I had 120 badges and was trying to decide between the legs, and some combination of the bracer, ring and cloak. Ultimately I went with the cloak and ring -- because doing the spreadsheet above, I discovered that the cost per gain across the board on the bracers was way too high for my tastes. I was very happy I made that particular choice because I knew it was the right thing to do having done the analysis.
A lot of people will give simple answers: the legs are great, get them first. But honestly, if you have the plate legs from Kara and a blue ring with crap stats, the badge ring might well get you much greater gains per badge spent than the legs will. You might have adequate legs, but are having major threat problems, getting the broach or the auto-blocker might be a better choice given your needs as a tank.
In other words, don't listen to anything anyone answers . . . do your own analysis and make an informed choice.