I have the same issue, I built my beds out of 2X6 material. Mine dropped a couple of inches in some spots, but my plants haven't cared. I would say mix in some new as you harvest plants, but don't touch anything if there is stuff growing there now.

Here is a link that might be useful: My Garden Blog

I originally built mine 8" deep, but it still settled to around 5" by the end of each season. Now, it's 14" deep, and still has at least 12" of soil to grow things in when settled - much, much better....

EG

Here is a link that might be useful: My square foot gardening blog

I have my pathways between the boxes mulched with wood chips. I added more chips in the fall and spring, and then I pried the boxes up, pushed some chips under the box edges, and then added lots more compost. I now have 2x6 framed boxes sitting over at least 9" of mix/compost. Next year, I intend to jack them up again. It saves on the lumber cost of building double or triple high boxes. I get the chips for free from a local arbourist, since it saves him the cost of tipping it at the dump!

the answer can be found in the book. try reading it.

If the bed has a dirt/soil "bottom", you should be fine.

Ha, I did 18 inches and mine's settled a whole 6 inches over the years. I plan on adding to it next spring. Don't touch what's there now. If you bury seeds more than the optimal level, they likely won't grow. Good luck.

Here is a link that might be useful: Sinfonian's garden