That is a very interesting question about the trap height, and I do not know the anwer. The "platform" mount is a fairly recent concept which is becoming very popular. In a normal replacement situation, I am sure no one is raising the existing trap. But logic tells me that the "floor level" should be the level the machine is sitting on.
I hope you don't mean a trap with the little cleanout plug on the bottom. Nearly useless, especially for a washing machine. If anything, you would like to have the ability to completely remove the trap for cleaning, but such is not a code requirement. You will clean from the standpipe, and there should be a nearby cleanout plug for the line.