We have quite a few clients and we run conversion tracking on probably around 75%.
It's a case of testing and refining with everyone as each niche is different and everyone does better in different positions.
For most clients we found the best positions to be were around 3-5 in terms of cost/conversion and volume. Some clients did best in the top spots, so testing is vital.
If you are in the top positions you can get a lot more tire kickers, which increases volume & CPC and lowers conversion rate which skyrockets the cost/conversion.
The main thing to look into is, what is the acceptable cost/conversion and try to maximise the number of conversions you can get for less than this. I'd prefer to get a conversion at a high but acceptable cost rather than let one of the competitors get it.