I think a lot of people tow at the ratings, and you can certainly do that, but there are a few things to consider...
First, make sure you are accounting for the full frontal area of the trailer. Many tow ratings specifically note that they do not account for that kind of surface area - which will impact towing.
Second, consider WHERE you will be towing. Being near limits in flat-lands at sea level is one thing. If it's a gas engine, however, you will lose power as you increase altitude. If you are headed into the mountains, you are going to want some extra power.
A lot of it boils down to your own comfort level. Our set up is less than 75%, so I never even give it a second thought. I could easily go higher, but doubt I would personally want to be more than 90% or so.
PS - just to make sure it is said somewhere in here, make sure you account for your vehicle load, as that comes off of your tow rating as well (with the exception of a possible accounting for a driver depending on the truck). Thus, if your rating is 6,600 pounds and you load 800 pounds of gear and people in your truck (over and above the allotted driver weight), your trailer now has a max of 5,800 to stay in the ratings.