I've got a 454 that I want to put a set of roller rockers in it. The engine isn't assembled yet. I just bolted on the heads. Do I need hardened pushrods and new guide plates?

Yes - to the hardened pushrods, I would recommend you use them in any performance rebuild. The roller rockers may require a shorter aftermarket pushrod anyway and most of the shorter are hardened that I've seen.
As for the guide plates - 'do you have to replace them'? Don't know - do the new rollers require the style guide plate you have? If you do need them - are your's worn in the guide area or are they in good shape and will work with the size pushrod you are going to run? These are all questions you should run by your cam/valve train supplier and get their recommendations for your application.