That is exactly where I find VMs helpful.
As far as Windows and Deployment Services is concerned, VMs are fairly transparent. VMs can help you test deployment settings up until you need to make sure that the hardware settings work.
In my company we had to deploy over 100 Windows Server installations over the network and VMs helped me iron out most issues with it. I say most is because there are a few things you have to keep in mind that VMs cannot test for you, namely the very beginning of the deployment process where the system has to first transition from network boot.
VMs cannot let you test hardware compatibility or that it works on a particular hardware configuration, because it isn't there. So keep that in mind that if some configuration or software has a hardware dependency, VMs will not be able to help you with that aspect. But if you want to test whether the Windows deployed with IE configured a certain way, and group policy implemented correctly and such, then VMs are useful and they make testing faster.