There are many times when it is worthwhile to create a custom machineKey for your web.config file. This is worthwhile on a webfarm but also worthwhile on a stand-alone server so that your machineKey remains the same after an iisreset or application pool recycle.
I've put together a simple tool that will generate a valid random machine key. It will create a validationKey and decryptionKey which you can copy and paste into your web.config file. This is random so there is virtually no chance of anyone else having the same keys.
I hope you enjoy!