I do believe that agent with professional attitude, well mannered are everywhere in the market (despite on which agency they are working in) and of course, there are bad apple in all agency as well, no doubt about that.
Having working in corporate environment for more than 16 years, experience taught me a very good lesson, whenever there are people they are politics, when they are politics, they are good and bad apple within as well.
In my humble opinion, it doesn't mean that only the top 5 or top 10 agency would have professional and well mannered agents, where else the rest would be differed.
Agents are human too, and when we are dealing with human, there will be tens and millions kinds of attitude and personality we are dealing with (some can be easy going, some can be calculative, some can take advantage on you etc)
So to answer your question, always observe the working environment for yourself, and to be frank, if you had yet to be working in any corporate environment, let me tell you this, in all industry there are people that does share, and some would keep to themselves. But I do believe sincerity is the key to success in this line of business.