It depends on a few factors.
First of all it depends on the scale of the event. More competitive events and markets drive the overround percentage down.
Next the companies client base plays an important part. If a company has customers that are not particularly price sensitive then there is a less need to be competitive.
What I do is go to an odds comparison site and add up the percentages to see who is the most competitive. Usually the smaller firms and independents are the most competitive but the chances of getting a decent sized bet are difficult.