There isn't a grid. If you have your photos fill the cells, they will automatically be aligned because the cells are aligned. (Right-click on your photo and choose Zoom Photo to Fill Cell, and then drag in the photo to choose which portion to display.) Otherwise if you prefer Fit rather than Fill, so that nothing gets cropped off, zoom in to 4:1 using the Preview panel, and then drag your photos to better align them.
This really did not work well for me. Zooming in on one page and not being able to see the other page is no help, If you could show the gutter half of each page, it would make it easier to line up the tops and bottoms. You'd think they would all align so the middle of the box was in the middle of the page, but that is not what it does. What I ended up doing is re-pairing images so each spread had similar proportions for each image. That looked much better.
Also for anyone exporting to Blurb, PROOFREAD EVERYTHING! Even if your book is correct, you can find errors in the book in Blurb, such as missing captions.
I agree that more functionality would be useful, Some, but here are some thoughts in the meantime:
To get all the photos/cells aligned, one option is to set the page type and padding on one page, right-click in it and save it as a custom page, and then use this custom page layout on other pages - there is a Custom Pages section in the page layouts. This way all your cells / photos would be aligned.
Also, while your method of cropping ahead of time to common proportions is an excellent one, you could also do this in the Book module by adding a photo, using padding to set the cell size you want, and then right-clicking and choosing Zoom to Fill. Finally, click and drag in the photo to set which portion is cropped off. Again you could save this as a custom page and apply it to others.
Finally, I wholeheartedly agree on proofreading - thank you for recommending this! I suggest that before users export to Blurb, saving their book as a PDF and printing it out - even on a document printer in black and white. I personally don't catch all my typos until I see the pages physically in front of me. There's something about screen viewing that causes me to gloss over things.