My understanding of grill block is that the car's designers have designed the grille openings for the hot extreme of ambient temperature, plus a little more, for safety. Accordingly, a grill block in several smaller pieces is often used, so that it can be added or removed in increments. The foam pipe insulation pieces mentioned above, are good in that aspect.
Personally, I wouldn't use any grill blocking once ambient temeratures are into summer range. And if you're using air conditioning it seems to me any amount of grill block is too much.
Here's some guidelines, albeit for a couple of hybrid vehicles, but it illustrates that a flexible amount of grill block is good strategy:
http://www.cleanmpg.com/forums/showthread.php?t=17560
http://priuschat.com/threads/2010-pr...trategy.62556/
(the second link used to have a much clearer graphic, but since the site rebuild a lot of images were orphaned. The text in the first posting still shows blocking percentages, though.)
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