This only applies to you if you have the Pages feature right under the header, and/or if your header is just text and not a full-fitting image. Unless you chose a ratchet header image that doesn't even fit the whole thing--that you want to align to the center. No problemo, this post will teach you how to align anything to the center.
FOR BLOG TITLE/HEADER
1. Go to blogger.com
2. Click on your blog title
3. Click "Template"
4. Click "Customize" (orange)
5. On the left, click "Advanced" (Although it's not that advanced. I can't really explain the label.)
6. Scroll until you see "Add CSS"
7. For centering image header, paste this into CSS box:
#header-inner img {margin: 0 auto !important;}
#header-inner {text-align:center !important;}
For centering header text, paste this:
#header-inner {text-align: center ;}
FOR PAGE TABS
Follow steps 1-6 from above.
7. .PageList {text-align:center !important;}
.PageList li {display:inline !important; float:none !important;}