My 2 cents...
You're going to have to compromise on something with 2 rows of seating, there's no way around it. Perfect setup would be 1 row of seating with the viewer's eyes at a height of 1/3 of the screen, but adding the second row requires adjustment for the best overall experience
For that, I would keep the screen at 30" from the floor, and use the 12" riser. If you drop the screen lower so you have the ratio you want for the front row, your back row will most likely be looking down
at the screen, making it uncomfortable to watch for extended periods. Looking slightly up
from the front row (in my experience) is not an issue, especially if you have reclining seats that will slightly tilt your viewing angle when you sit back anyways.
I made a very similar compromise in my own theater with 2 rows. the screen is slightly higher than recommended, so front row viewer's eyes are also below the 1/3 mark. This puts my back row viewer's eyes almost directly center with the screen, which is perfectly comfortable (for me at least, to each his own).
Also, don't forget your speaker placement. You never mentioned what type of screen you have or speakers you're using, but going with a back row will require moving your LCRs higher than normal as well (and a center channel below the screen with 2 rows of seating is normally considered a no-no since you'll basically get no dialogue to the back row).
EDIT: I second what Big said about the booster for the rear row. I actually added different feet to my rear row recliners, which only added an inch or 2 of height, but it made just enough difference for my vertically challenged fiancé.