If you add the tag "$:/tags/AboveStory" a tiddler, it will always be displayed at the top. However, this only displays the tiddler's text content, without the tiddler frame, title, subtitle, or toolbar. To show the complete tiddler display, you can do the following:
1) Let's assume the tiddler you want to "pin" at the top is called "MyStuff".
2) Don't tag "MyStuff" with $:/tags/AboveStory.
3) Instead, create another tiddler (e.g., "StickyTiddler") and tag it with $:/tags/AboveStory.
3) Then, in that tiddler's text field, enter:
{{MyStuff||$:/core/ui/ViewTemplate}}
enjoy,
-e