From a comic lettering web site [?!?]
Comic book lettering has some grammatical and aesthetic traditions that are quite unique. What follows is a list that every letterer eventually commits to his/her own mental reference file. The majority of these points are established tradition, sprinkled with modern trends and a bit of my own opinion having lettered professionally for a few years now. The majority of these ideas have been established by Marvel and DC, but opinions vary from editor to editor, even within the same company. I'm often asked to bend or break these rules based on what "feels" best, or more likely, the space constraints within a panel.
As a letterer you're eventually going to see scripts from writers who don't know these standards, aren't interested in them, or just have poor grammar all around. (Although I find the best writers ARE well versed in these points.) It'll be up to you to spot and fix these in the event that the editor misses them.