It doesn't affect it at all. I was concerned also until I tried it out in the store. Here is mine from the side open.
I just want to mention this: I just got my hardshell case today for my black macbook and put it on. I also have a white macbook to compare it to. Although you can tilt the screen past 90 degrees, it does not go to its full reclining angle. I was fiddling with the bottom case for 30 minutes trying to slide it under the hooks of the vent properly. I took it on and off about 7 times. After I finally figured that I had it installed correctly I took a protractor and measured it up against the white macbook. This is what I found about how far the screen reclines:
White Macbook (no hardshell case) ~ 142 degrees
Black Macbook (with hardshel case) ~ 138 degrees
Although its only 4 degrees, it is a noticeable difference when comparing it to another machine. The macbook is still very well usable with the hardshell case on and you will only miss the 4 degrees of incline when you are using the macbook on the ground in front of you or in similar positions. Your viewing angle will not be hindered if you are using the macbook on a desk or on your lap.
Just a side note, I have the invisible shield installed on the white macbook and tried installing the incase hardshell case but the bottom case didn't seem to fit right, although it did fit.