Most are not laminar flow. I believe this is to do with the difficulties in manufacturing a flawless surface (no exposed rivet heads, panel edges,etc).
I drive a composite aircraft with a flawless wing that is not designed to be a proper laminar flow wing. I still clean the bugs off the leading edges if there has been a noticeable build up. This is as much about evening up stall behaviour as maintaining performance and is based on guesswork rather than empirical evidence.
I'm far more concerned about dead flies on my prop though. I've never noticed degraded performance due to flies on a wing but a prop covered in flies is less efficient and I'm sure it could be proved. My prop gets proportionately far more fly coverage than my wings. I clean my prop whenever I think its needed as a fuel saving measure.