Kendra Little looks at cases when a query uses multiple tempdb data files:
As you might guess, things may not always get evenly accessed, even if you have evenly sized tempdb files. One of my queries did a select into a temp table. Although it used all four tempdb files whether or not it went parallel, there were more file_read events against the first tempdb file than against the other four.
It’s an interesting look at this specific question, but also as pedagogical technique.