Well since its coming from a garden supply the rocks should be clean, soaking them for a short time will get any excess dust etc. off the rocks. Unless they are dirty IMO I don't see a reason for the power washing.
__________________ “Why does Sea World have a seafood restaurant? I'm halfway through my fish burger and I realize, Oh my goodness.... I could be eating a slow learner! ” -Lynda Montgomery
Personally i would just hose them off and scrub with an algae brush or something similar, but to be really thorough soak in warm water for 24 hrs first and then spary and scrub. You can never be too careful! I love those rocks!!!! i have a ton around my koi and goldie pond. Good Luck!