Generally, what you heard was correct. At the very least, you can loose root. At worst, you can screw up your phone....somewhere in between is likely where you would end up...in some partially rooted state.
Now with the philosophy of it all. Why would you want to accept the OTA??? Once the developers get their hands on a true official version of 2.2 they will push out a ROM based on the new update. There are many times they get their hands on the official code and push the update out before Sprint does. Anyway, if you unroot and then accept the OTA you will be stock and unrooted and potentially be that way for a LONG time. Why? Because they will have not found an exploit to root the new update and it takes time for them to do that. And even when they do find an exploit, it takes even more time to build a one click rooting method. The developer's priority is not with the people who chose to unroot an accept an OTA. Their priority is pushing out ROMs to people who chose to remain rooted. The newest version of the EVO is not rootable with a one click method and has been so for almost 2 months. Are you prepared to be without root for potentially monthS??? If that doesn't bother you then it is no big deal. But it is something to think about.
PS...EVO guy here so weigh my opinion accordingly.