I do each panel totally before moving onto the next. In this case though I'm waiting on doing the dialogue and bubbles until after I finish the rest of the pages.
So it normally goes:
Rough sketch (usually 2 of these)
Sketch (each character\backgrounds separately, 2-3 times, or more if needed)
Ink (characters first, backgrounds second)
Repeat for each panel as needed
Dialogue and speech bubbles
My way isn't the most efficient but makes the most sense for me.