i have always woundered that, when i hauled mine home this winter i just cranked them enough to hold it side to side, and put another one over the swingarn and through the tire for sipport.
but i know people that dont like compressing the forks becouse if you have them cranked down enough that if you hit a good size bump in the road that it can put too munch pressure in the forks and even blow the caps out (on smaller bikes)
75% may be a bit much, but you're definitely not gonna blow seals or caps out of the forks. Besides, if you hit a bump that big going fast enough to completely bottom out the forks, you are driving way way too fast considering that you have a 400 pound bike in the back, and you are not paying enough attention to the road ahead of you.
I crank the tie-downs tight enough so that the front forks are about 2/3 loaded.
Ultimately, as long as the bike doesn't move you're ok, but you have to keep in mind that just becuz the bike doesn't move when you push on it, that doesn't mean that the bike won't shift around when you hit a big dip or bump on the freeway goin 70mph. I learned that yesterday while taking my bike to/from the track for the first time in a buddy's Ford Ranger. Some of Interstate 5 (north bound in southern california) is really crappy, big whoops and rises and such.
As for the rear, I looped the tie-down strap through the passenger peg brackets. I cranked the tail about 1/2 way down. Again, plan for bumps and crappy roads.
One more thing, and you may already know this, but it would be a good idea to take small rags, socks, or just strips of fabric and tape them to the tie-down straps where they pull down across the front fairings. That way you won't scuff the paint. This applies if you loop the front tie-down straps on the clip-ons behind the big plastic high-beam and on/off switch housings.