They should, in all things that include a summary, web article or otherwise. When a tl;dr is at the end of something, it is tl;ar (too late; already read). It drives me nuts, it defeats the whole purpose.

On a related note, Romans wrote sums at the top of a stack of numbers they were adding. http://www.etymonline.com/index.php?allowed_in_frame=0&searc...

Summaries at the top, always.