What they said- run a real calculation!
Trom both a comfort & efficiency point of view you're better off being a ton low than a ton oversized. If undersized it'll dry the air out (the "latent load") on the few hours/days where it won't keep up with the load. If oversized it'll chill the temp faster (the "sensible load") without drying it out as much.
In the mid-Atlantic region latent loads are typically several times the sensible loads, so if your Manual-J calculations (which factors in both sensible & latent load), ends up between sizes, erring to the small side is almost always the right thing to do, since a 100% duty cycle improves the latent load cooling factor. On those hazy days where it's just a wicked-muggy 80-85F outside an undersized unit will definitely "feel" better since by running longer cycles it'll be dealing with the humidity better.
Also, Manual-J isn't perfect- there's some fudge-factor oversizing built-in. If you're planning to do any significant upgrading to the insulation or tightness of the house, an ideal-size for the existing condition of the house may prove oversized after envelope upgrades. If that's in your plan, be sure to run the Manual-J on the anticipated, not current condtions. In the size range of compressors you're talking, undersizing it a half-ton from Manual-J is usually "safe" all the way around. A full ton undersized might not always cut it, but it should most of the time.
Undersizing generally reduces wear & tear on the unit too, since the bulk of the wear is at startup, and the fewer cycles it experiences, the longer it will last.