No, I use a front rack. Something over the tires would obstruct my view of the road too much for me to be comfortable with it.

Since you're not taking food, the weight is relatively low, and probably won't make a big difference. It can be a shock, though, when you pedal down hard on an uphill and feel the bike move up instead of forward.

If you decide to move anything to the front rack, make sure the weight is evenly balanced on each side, and that the surface area is close to equal. My mat is exactly the same size and weight as my tent, so I lash one on either side of the front rack, and off I go. I used to use a mat that was larger than my tent, and whenever I'd get above 18 mph, the increased wind resistance on that side made steering much more difficult than it needed to be.