its not really necessary to encrypt the drive the operating system and mining software run on. the mining software merely mins for bitcoins and tells the mining pool what address to send TO. unless you plan on running the bitcoin wallet software, along with the wallet you are sending to, on the same system, its not advisable to encrypt. though, its also not advisable to run bitcoin wallet on on the same system as a miner.

also for the record, in case anyone is reading and wondering, encrypting a volume and running bitcoin with your wallet, does not protect you. when you decrypt the data, and it is accessible, its just as easily intercepted by nefarious parties. if you have a wallet.dat INSIDE an encrypted volume that is NOT mounted and decrypted, it is "safe"... until you decrypt/mount the volume. you should ONLY access your "savings" wallet within a known virus-free operating system. you should not access it elsewhere.