I always put the wheels at the back as this is the position I use to get the boat off the trailer. I have had the occassional embarasment when I have tried to lower the mast with the wheels just back from the centre beam. You get about 1/2 way down and the boat will rock back and point the nose in the air while trying to run away from you.

The grass in the photo is a nice change for me. I usually launch from a pebble beach and I put foam blocks under the noses to protect them.