I think you're worried about doing a Full Bleed book, (where your designed page extends all the way to the edge). Will some of your design get trimmed off the sides? Will the inner margin and binding of the book hide some of your page on the inner edge?
Yes, these are issues you have do deal with for a full bleed page. I had to process every one of my pages after the fact, to put in that 1/2 inch margin. And it was a bit of a pain. The 1/2 inch margin did perform well, though.
I'm considering printing all future pages in Blurb books, but I've done some 2-page designs lately, and based on my last book, a 2-pager would be problematic. So this is what I'm thinking of:
Instead of doing full bleed, shrink the image down and just have a blank border all the way around. The Blurb software will do this for you. Here's a screen shot of me playing around a little after reading your question:
If you set up the first page, using the Picture Layout that has a single large picture, the software will shrink the image for you. So nothing should get trimmed. This default page looks like it gives about 1/2 to 2/3 inch all the way around.
If you go to Edit Layout, you can shrink the picture even more if you feel that is needed.
And then if you do AutoFlow (sorry, I didn't circle that in my screen shot) it will apply that image size to all pages as it automatically adds them to your book.
So, that's one idea to circumvent the margin and page trim problems. It gives the blank border all the way around, which you may or may not like, but it lessens the trim and binding problems.