It depends. If I write up a big, proper paragraph explaining what a block of code does, then I punctuate it properly, like any other piece of proper writing. OTOH, when I just comment a single line of code, then I don't.
Why? - Similar to why I write emails using proper writing, while I might use shorthand sentences in SMS messages. In one case I'm sitting down to write a proper block of text, so I just automatically "do it properly", while in the other it's just a brief note to get a point across.
Real examples from my code:
Quick note comment:
// check for vk_enter
"Proper" method documentation:
// This method sets up a workspace tab with the given name. Each MDI window has a parent
// workspace specified when it's saved. The code which loads each MDI window then point it to
// the correct workspace.