Use your logic. Proper and experienced engineering design dictates that the vapor spaces of a condenser and its corresponding downstream receiver should be connected by an equalization line in order to ensure complete and efficient gravity drainage of the condensate into the receiver.
One valve would be insufficient to block the condenser from the receiver; you would need another valve on the equalization line as well.
But Latexman's two reasons are even stronger. Why in the world would you require a block valve between the condenser and the receiver? You fail to give us the most important basic data: this is a refrigeration loop (circuit) or part of a compressed gas liquefaction system -- correct? Since you don't tell us, I have to assume something. You should furnish all the basic data if you want a specific, correct answer - and identifying the process is part of the basic data.
As it has already been said it all depends on the system: its size, the service it accomplishes, gas cleanliness, whether there are two parallel condensers, and so forth. As I usually recommend: follow Mr Montemayor's advice.
