Position player.....yes, throw hard....within reason....lol. Nobody will care if you throw it to the backstop or over the fence a couple times....as long as you don't do it every single throw. They realize that the player knows his velocity number is important.
Hitter....don't go crazy during BP trying to hit HR's...especially if you're not a HR hitter. Most showcases will take a velo reading at some time...either off a tee or at the start of BP. Go ahead and swing as hard as you can during that part of the hitting drill. Now if you're a 6'4, 240# kid who can take it out of the park...by all means, do it, as that's likely what they're looking at that sized kid for anyway. Smaller kids, such as MIF who try to swing for the fence, just typically end up popping a bunch of balls up....or worse yet, swinging and missing.
Running....yes, go ahead and cheat a little...but not so much that it's noticeable. You don't want to be the guy who they make stop, come back and run again because you cheated too bad. Easiest way is to just turn your front leg just a little bit...instead of having both feet in line with each other....they typically won't have an issue with that.