This is a difficult question, and has been heavily debated. At the end of the day, it is a question of personal preference. This is an example of the debate on coffeegeek.
I use a convex tamper right now, for a few reasons:
- The bottom of my portafilter basket is convex, so a convex tamper actually makes the puck a consistent (albeit curved) thickness.
- The convex tamper is also believed to push the coffee grounds up against the side of the basket, which helps prevent channeling. I use a La Marzocco double basket, which necks down so also tends to seal the edges during tamping, so I am not sure.
On the other side of the debate there are just as valid reasons, so it is a tough one.