In the past, we've advised people on how to return their EVOs to stock unrooted condition before returning them to Sprint for repair. A few days ago Sprint's policies regarding dealing with repairs for customers with rooted devices was posted on ACSyndicate, and evidently you don't have to bother with unrooting.
While this might be old news as the policy has been in effect for some time, it's not exactly been in Sprint's interest to tell you their technicians are supposed to follow a set of guidelines that allow you to bring in a rooted device in for service.
The short of the already short Sprint policy document is that if a technician suspects a device is rooted they can advise the customer on the danger of rooting, and then can go about re-loading the stock software to start hardware testing.
xda-developer folk-hero Ropodope decided to test this out when his stock battery decided to degrade significantly on his rooted HTC EVO 4G.
After informing the Sprint technician of their own policies (aided by a printout,) he was able to get a replacement battery on his rooted device.
While Ropodope's experience was fairly drama-free when armed with Sprint's own documents on handling rooted users, without them others have reported issues. Your experience may vary.
It's also important to note that if you bring in a rooted device, policy states they will return it to stock, erasing data and potentially unroot the thing. So make a nandroid, back it up to a computer, and be ready to re-root later.
Also worth noting that the Sprint policy document has been floating around for a few months now. As it was for intended internal use only, your possession, re-production, and subsequent use of it may actually violate your Sprint terms of service. Downloading it could be considered intellectual property theft, and using it could be considered abusive behavior.
Probably not, but better to know what you could face if you end up dealing with an irate Sprint technician that doesn't want to take the six minutes to return the phone to stock.
So yeah, there's probably one less reason to not root your phone.