yes i rooted my evo when it was Version 3.70 i used unrevoked method and i have to say i believe its the best method to rooting, have a look at this post:

http://androidforums.com/evo-4g-all-things-root/194918-rooting-dummies-extras.html

It will tell you everything you need to know about rooting!

 

As darkworld260 said, yes you can root with 3.70 (more importantly, hboot 2.10). Are you already rooted, though? If so, you can't just accept the OTA update. You must either unroot, then accept the OTA update, or update manually (Rooting for Dummies - section 6 under after rooting). If you update via OTA, your hboot version will update, and all previous nandroid backups will be useless. Of course, if you're not rooted already, forget all that. You can accept the OTA update, then root just fine.