On Basic Brewing Radio I think they said 'in general, if you have an umbrella your beer should too.' However, that may have just been a precaution... my last brewday my pot was in the shade for mashing and cleanup but not for boil (of course) and it tastes fine.
That doesn't really make sense, as the boil is isomerizing those alpha acids, but if it works it works... (and I admit I love talking about 'isomerized alpha acids' because it makes me feel smart

)