It is more and more important for practical reverse-engineering. It is now present in malware, the example of Stuxnet, Flame and others are quite typical of the usage of cryptography in such context. And, it is also present in most protection schemes because a lot of techniques use cryptography to protect the code and data. Just consider software such as Skype or iTunes which are relying on cryptography to protect their protocol or to hide information in the executable.

So, indeed, it would be really a problem if you do not understand a bit cryptography when reversing. And, by "understanding cryptography", I mean at least to be able to recognize the code of classical cipher algorithms at assembly level such as DES, AES, SHA-1, SHA-3, and so on. And, also to know classical flaws and cryptanalysis techniques for weak crypto (such as frequency analysis).

A good way to learn about the cryptography needed for reverse-engineering would be to implement (with the help of existing codes found on the Net) your own cryptographic library with classical ciphers and look at the generated assembly. If you do not have the patience to do so, just look at the crypto-lib of OpenSSL, get it compiled and look at the code and the assembly.

Of course, more you know about it, more you will be efficient when facing it.