Personally I feel like you can break the engine in any way you want to but, it's all about how long you want the motor to last! Seems to me like slow breakin would help to seat things altittle better until you know for sure everything is together right.
I'll tell you want my dad does with his new boats that he gets every year. He goes out and runs the crap out of them until he is comfortable it's not going to loosen up anymore. He tells me that he thinks they run alittle better like that. He has no way of knowing for sure if it really helps any but, he does have lots of experience with boats ets.. So I tend to believe him about this atleast.
If you want to gets som good info about all this then I would look up some post by Grumpyvette and see what he has to say about the subject. This guy has tons of info about motors and such.