I have an outline for what needs to chappen in a chapter, which I then break down into a page-by-page list of what happens (like, "Page one, main character walks through the woods. Page two, he spots something weird on the side of the road. Page three, he goes to investigate", etc.)
I then take that and turn it into thumbnails - and write the first draft of the dialogue next to the thumbnails. That allows me to control the pacing, AND control the placement of the speech bubbles. Always, ALWAYS plan out where your speech bubbles are supposed to go before you draw the page. This allows you to avoid crowding your characters with awkwardly placed bubbles, and lets the page flow better.
My panelling style is closer to manga than it is to the strict grid-pattern, because I like to play with the size and angle of panels to imply motion and passage to time. Very tall, large panels encourage the reader to move their eyes vertically across the page, and can be effectively used for vertical motions - going up/down stairs, falling, implying that there is someone above the characters looking down at them, etc. Small, overlapping panels look like very quick snapshots of things happening at the same time, and create a kind of staccato rhythm in the reading experience. Very wide shots imply a longer passage of time, and work best for establishing shots of environments. Panels with a lot of space around a character drawn quite small imply a sense of space, or silence, or loneliness, etc.
In part 3 of Grassblades, I played a bunch with filling the gutters between the panels with stuff too:
Here, the darkness that turns up on the first page (page 11 of the chapter as a whole, IIRC) spills out between the panels on the following page, connecting them together and implying that something unsettling is happening, and on the third page of the sequence, the darkness turns into a giant demonic entity with a creepy snake-tongue. The darkness (I've been referring to them as "ink blots" in my head while drawing these pages) then flows between panels throughout the rest of the sequence with the demon.
There's lots of stuff you can do with layouts and panels beyond just showing what's happening.