You should run the blocked crossover pan whenever you are running an aluminum intake. it cuts down on the heat the manifold sees and reduces the chance it will get distorted.
Kind of hairsplitting, it will get plenty hot even with a blocked pan installed.
Some say that a blocked crossover will lead to increased ring wear as it will take longer for the engien to heat up. I think that is crap. Engine gets plenty hot, plenty quick from what I have seen. Maybe if you just let it idle to warm you would have an issue. I let it warm from about 30 seconds, then back out and take it easy until heat builds on the gauge, doesn't take but maybe a mile to get to 150, 180 after about 2-2.5 miles.