I've been reading "How to Make Webcomics" as well as doing a lot of reading/questioning on the Halfpixel and Blambot forums in preperation for my eventually starting a webcomic.

I've also been reading Sheldon for years now and am now looking at it with an eye towards style as well as just enjoying it which brought up the question -

Is there a formula or sort of rule for switching between speech bubbles/ no bubbles and frames/no frames? Is it a white space thing or a breaking up of content thing or is it just a what looks good at the time thing?