Just make sure to save the updated machine image afterward. :-)
The alternatives discussed here are for when you don't have and don't want to pay for a Windows license, and are unwilling to break the law and pirate it as part of your job.
My (completely legal) copy of Windows decides that it's not legit every time I upgrade VirtualBox, and sometimes at random times in between. Each time I have to dig out the disk and go through the authentication BS.
If I actually used it for anything other than occasional browser testing it might be worthwhile to jump through those hoops, but I don't. Not to mention that (since I only need it once in a great while) each time I use it I have to go through downloading a zillion megabytes of software updates and reboot multiple times.
Also, I don't have to keep it around on my (always full) main drive or run it off a slow external drive.
The two cents an hour cost for the AMI is well worth avoiding the hassle, for me. Your use case may well be different.