Fully agree with Rich. Depends if you want to go anywhere in the heart of town (as most people from out of town do). Some museums such as Air and Space do put your bags through X-ray machines and they have metal detectors (which 8/10 are not active). Others like the art galleries only open any bags you carry and poke around with a stick (not necessarily logically either. They searched my wife's little purse, yet I'd standing right next to her with a photographer's vest with bulging pockets and they just waved me through). Having any knife or even scissors is prohibited in most such buildings, whether the screening is thorough or not. Places like the Old Post Office (which is a mall) do have a working metal detector and will not allow you in with any knives.
Barring any building with it's own restrictions, the rest of DC has a 3 inch blade limit. Yes technically § 22-4514 requires possession "with intent to use unlawfully against another" but this intent is often presumed by the mere fact that your knife is larger than 3 inches. Plus I'm told local police can be a tad...thick, so I'd avoid any unnecessary risk while walking around.
