When I started Liquid Stealth (examples in my gallery here), I too, didn't want to cloud up a composition with a lot of speech balloons.  As I started with narration it was easy to squeeze that at the top or bottom of frame. But once, dialogue came into the story, the text had to go somewhere and it had to easily correspond to the character speaking. I tried doing the transparent bubble to minimize impact.

As Jenx said, plan less detail for where its needed or you'll need to blot it out. Text without the bubble often just ends up being hard to read. 

In the future you could invent hot spots where the bubble would pop up only if you hold your cursor over the character and disappear when the cursor moved away. 

Really advanced would be to cue a sound bite to play when you click on a character then your image would be text free.